When I had mushrooms in with him, the cowrie never went towards the mushrooms. I have since given the mushrooms to someone though, but he does not touch any of my other corals incluing zoos, bubble coral, caulstrea, finger, xenia, and my hard corals.
They are decent algae eaters. I really do not get much algae at all, so I could not tell you for sure. I will tell you one thing though. He likes to go after the nori I put in my tank for my LMB. He really moves towards it when I put it in there. Today, I was amazed by how fast he got over to it..
All in all, they are very interesting to watch and I am very happy that I bought mine when I did.