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"
Status: Known Issue
We are aware of this issue and have it logged internally but either we do not have a fix ready yet or we are not planning on addressing the issue in the immediate future.