I have this blue stuff that is kinda clear but a very bright blue on parts of my live rock. I’ve seen it in 3 places. Doesn’t look like a sponge to me and doesn’t seem like a tunicate either as it’s not growing and as far as I can tell it’s not pushing water out. rock is from a tank that is about 15 years old as well as a few pieces of rubble from the ocean. The spot photographed is 3/4” long