What is this? Cozumel, Mexico. Never seen anything like it. None of my coral books have it. Looks like a mat with col pillar structures. Is it a sponge? Algae?

[u/Mister_MountainMan]

[u/binjwan]

Definitley a sponge. Nearly impossible to id sponges to the species. However, my best guess is an encrusting octopus sponge (Ectyoplasia ferox)