Just as the oceans have a bounty of medicinal fish and plants, so too does the land, in rainforests, deserts, all over. Obviously, merfolk would not generally be able to waltz deep into a rainforest to collect plants.
Given that merfolk have limited access to metals/tools, they would have a hard time quarrying anything but the softest rocks. However, powerful merfolk would clearly prefer a palace built from marble over one built from coral. This would surely be expensive beyond measure, but that only adds to the prestige value of the result. Conversely, merfolk would have better access to pumice and other volcanic rocks emanating from steam vents. As you know, pumice is valued on land as a light, porous rock useful for personal grooming, soil filler, etc.
Land walkers would surely have knowledge of principles and processes which occur more readily (or exclusively) out of the water. Because they lack the benefit of buoyancy, land folk have been forced to make heavier use of leverage, leading to wheels, levers, and the like. Conversely, merfolk would have much better, even intuitive understanding of fluid dynamics, including pressure, turbulence, laminar flow, venturi effects, and other things which are useful for building above-ground devices of varying complexity. Of course, they would describe these phenomena in medieval language, but that does not preclude a kind of formal knowledge of them which can be communicated across species.
Merfolk would be challenged to weave fabrics underwater. Perhaps they could use coastal areas to harvest reeds and fibrous plants, but surely they could not grow vast tracts of cotton and other textile plants, and looms would be exceedingly difficult to operate underwater. It would also be more challenging to tan and dry hides, and bringing leather hides underwater probably wouldn't be useful anyway. Plant-based clothing could likely survive and would surely have luxury value even if domestic merclothes were more practical.
On the one hand, we must ask: "What does a mertoilet look like, and how does it work?" Presumably, elimination simply occurs in some area where the currents take away and disperse the results quickly. On the other hand, most plants are growth-limited by the availability of phosphorus, nitrogen, and other elements commonly occurring in animal waste. It's possible that merfolk have domesticated large fields of seaweed, and that they fertilize them by also keeping their food fish in the same area, possibly also using the fields as their communal toilet.
However, water will tend to disperse nutrients, and simple tide action will guarantee that the water doesn't just sit still and let nutrients settle slowly over the course of hours. Perhaps merfolk learn how to fertilize seaweed manually by burying wastes in the seabed near the plants. If so, then one limit to seaweed yields will be how much "fertilizer" they can apply. While medieval Asians were applying "night soil" to their fields, this was far less common among Europeans, so it is plausible that your land walkers would rather sell their waste to the stupid merfolk than let it build up in their cities and cause disease. They could even build primitive sewer pipes that bring it directly down into shallow coastal areas, and thus a coastal landwalker and merfolk city could operate symbiotically.