This is a harmless Scypha sponge. They are very small, and usually found in our overflow boxes and down in the sump. Attached to rock, glass or acrylic or even your plumbing, they may be about the size of a grain of rice or perhaps larger. These little filter-feeders are reef-safe. Also called Pineapple Sponge, or Q-tip Sponge.
Photo by Debi (dc)