The short answer is most likely.
There are two selection pressures that control the color of human skin: the amount of UV light received and the need to produce vitamin D.
Darker skinned human races have invariably formed near the equator. This makes sense, as the Sun's rays are generally much more perpendicular to the Earth's surface, resulting in high sun exposure. The addition of melanin protects the skin from UV damage.
Lighter skinned human races invariably form near the poles, however, this isn't due to simply to the lack of UV exposure, but instead, due to the need to produce vitamin D. Because there is less UV exposure near the poles, and on average just less sunlight in general, human races near the poles have faced a selection pressure opposite that to those near the equator. Namely, they have decreased melanin production to allow more UV into the skin for calciferol production.
This can be seen in the health data as well when humans are moved out of the environment for where they are adapted. Namely, when races from higher latitudes move near the equator, they have higher rates of skin cancer. They also dynamically adapt (i.e. they tan) to the new environment. Conversely, vitamin D deficiency rates are much more prevalent in those of dark skinned races when they move to higher latitudes (i.e. closer to the poles).
So what does all of this mean for your merfolk? Well, if they have to produce vitamin D then they are likely to be much lighter skinned. Water is an extremely good blocker of UV light, as a large amount of UV is reflected at the surface, and what makes it into the water itself is quickly absorbed and scattered.
Since merfolk live predominantly underwater, they are not as likely to be exposed to UV, so there would be no selection pressure for them to develop darker skin. However, if they, like humans, need to produce vitamin D, then they will face a strong selection pressure to develop lighter skin in order to maximize their usage of the available UV.
This would create a problem though, in that merfolk would not be able to spend much time at all near the surface of the water where UV exposure is the strongest. Humans with albinism (a complete lack of melanin production) are forced to take extensive measures to protect themselves from sun exposure, even for small amounts of time in the sun.
It is important to note, however, that even with complete lack of melanin production, albinos are not "snow white" in complexion. The color of human skin isn't simply a function of the amount of melanin available, but instead comes from complex scattering interactions through multiple layers of skin. Skin is actually quite translucent, and a lot of the color you're seeing is from light scattered through blood and viscera under the skin. It's one of the reasons a huge amount of research was required before CGI rendering of humans looked correct and not "rubbery."
That said, the ratio of Red to Green in human skin is highly conserved, regardless of their skin color, and computers can be easily trained to pick out humans in images by this ratio, regardless of their skin color, as even though blacks and whites look very different, if you look at the RGB ratios, you find the relative values of the numbers are more or less the same.
So if you want your merfolk to be "milk white," you will likely need another mechanism to cause it. However, you can use any number of selection pressures to push this. Blue eyes and blonde hair became prevalent in European nations not because they provided any sort of survival advantage, but because there was a strong selection pressure for them as their "exoticness" made them desirable. Similarly, you could say at some point in the merfolks' past, a single mutation was seen as "beautiful" to the locals, and strongly selected for from a sexual standpoint.
Or, just as easily, you could come up with some sort of survival situation where being "milk white" provided an advantage. Fish often have silvery scales that reflect light in order to confuse predators, especially when the fish are in a school. So perhaps your merfolk faced some sort of predator that was confused by "flashing white."
Others have also pointed out the bicolor pattern that is prevalent in fish. This pattern is not simply to confuse predators; it is a camouflage, which benefits both predator and prey. When seen from below, a bright white body creates no silhouette with the bright sky above, and when seen from above, a dark body creates no silhouette with the dark ocean underneath. Such a pattern has obvious advantages if you are trying to hide, as well as if you are trying to ambush. So if you are ok with your merfolk having dark backs but light bellies, then it wouldn't be out of place at all, as it would give them a survival advantage.
One thing to keep in mind, however, is that merfolk are presumably intelligent. And evolution doesn't work the same way on intelligent species as it does other animals. If you have an animal capable of creating culture, cultural evolution happens far more rapidly than natural selection, and so it is far more likely that your intelligent merfolk would have developed bicolor patterned clothing long before any selection pressures kicked in. This is all the more likely if your merfolk evolved from humans who spent long periods of time in the water, rather than "climbed up the foodchain" like humans did.