“Found this in the Mississippi River while kayaking. I think it’s some kind of fungus but I’ve never seen this in my life.”
It’s Bryozoa — a type of invertebrate animal. These are aquatic and colonial moss creatures. The colonies of Bryozoa can cover the square of more than 10 sq ft. Their colonies have many forms: some grow on reachable surfaces (stones, shells, or seaweeds) in the form of crusts and lumps, while others can have forms of fans, bushes, and sheets.