Water deprivation merfolk whump.
Are they suffocating out of water? Or just uncomfortably dry? Perhaps they're pinned to the ground by gravity, unaccustomed to their own weight. Perhaps they try to crawl in search of water, scraping up their elbows, belly and underside of their tail as they drag themselves across dry ground.
!!! Also consider: all fish have a thin slime layer protects them from sun damage. Should merfolk have a comparable slime layer on their scales it would make them even more vulnerable to heatstroke and sun damage if it were scraped off in the process. If they make it back into the water it also makes them more vulnerable to parasites.
But now imagine, a well meaning human/land being stumbling across the struggling merfolk and trying to help out by dragging them to a nearby body of water! Only... the merfolk is from a saltwater colony, and once they are dumped into freshwater they will nonetheless slowly weaken and succumb to the wrong environment!