The colorful coral tiles generally seem to be considered shallow water by most of the game, but technically aren't GROUND.OCEAN_SHALLOW. So seaweed spawns there naturally, but if you remove it with a trawl net you can't replant it there.
Steps to Reproduce
Use a trawl net to obtain a seaweed stalk Attempt to place on "coral water"
Use a trawl net to obtain a seaweed stalk Attempt to place on "coral water"