This very cool little creature is a Lettuce Nudibranch. They come in specifically to help mow down nuisance algae such as bryopsis (visible in the foreground). I purchased six of these for my reef tank, and each was about 1" long, and about 3/8" wide. Delicate and very light, they were introduced into the tank with the pumps off. Each one was held gently against the rockwork for about 10 seconds until it held on. Every day, I'd find one or two of them in the sump or refugium, and I move them back up to the display tank. Covering intakes to powerheads would be wise. Lettuce Nudibranchs are not known to last long in our tanks, but can be a helpful creature when needed. Reef-safe.