How magical is your world?
It's hard to give a solid answer without knowing how the magic works so I'll keep this general.
It's not entirely clear if you want individual coral polyps to be extremely large so a single skeleton can be used for some purpose or if you want to radically increase the generational cycle of a colony of coral polyps, so you get lots of raw reef material to work into some other form.
Either way, you're going to wreak havoc.
Giant coral polyps would involve creating new species and/or changing existing ones. We don't have any track record of deliberate speciation so the closest analog I can think of is species introduction (i.e., "invasive species"). Generally, introducing a species to a new environment where it has never existed before is a bad idea. Other species are almost certainly going to be lost in the process, possibly many other species.
Even if you limit yourself to "just" speeding up the life cycle of existing species, you run into problems.
A normal coral polyp has a lifespan of a couple of years. Based on your description you want "decades" of growth in "a few months". Since that's not exactly precise, I'll assume you want 50 years of growth in 3 months which works to a nice 200x increase. So, where is all the food and other resources for all of that coming from? Assuming you're not handwaving it all with magic (and given that you're even asking, I'm assuming that was not your plan), then you need to take those resources from the regional environment and, again, things die on a massive scale.
Once each of those polyps has done its thing, it dies. That means a lot more food for decomposers. On the scale you're describing, most ecosystems cannot handle it. Again, if you're not handwaving it all away, you're throwing the environment into chaos and things die (making everything worse).
Any time you increase the biomass of a single part of an ecosystem at a rate significantly higher than normal you will almost certainly have an ecological disaster.
If you're doing all of this with magic, then you can magic in the resources and magic away the waste. You can magically prevent species and habitat loss. You can do whatever you need. If you're going to use magic, embrace it. Or not, maybe you want to explore the consequences of this magic. Then just accept that your merpeople are going to destroy the environment and their descendants will not speak kindly of them. Assuming they’re not among the casualties.
Edited to add the following
Addendum: scale matters
There's a huge difference in impact between any random merdude can casually make Australia's Great Barrier Reef grow at 200x vs a small number of highly trained/talented/gifted mermages can, with effort, increase the growth rate of coral over 10 square meters. If you limit your scale, you limit the damage.