This is a great example of an original yet game-fitting mob. The idea of a "sea cucumber" would be laughable in any other game, but with Minecraft's silly atmosphere I can see it working quite well. Clay farming is a great way to make the mob useful- clay is hard to find, and rarely used when building now. I wouldn't have it drop primarine pearls or fish though- leave that to guardians and fishing.
I wouldn't be against adding seaweed to the OP, just make sure to connect it with the cucumber or else it may get marked as wishlist.