I noticed these yesterday on the base of my Acan. The only other inhabitants, (that I know of) in the tank are trochus and nassarius snails, skunk cleaner shrimp and a orange spotted goby. They are only on my acan and nowhere else in the tank. Any ideas on what these could be? I started the tank with dry rock, so if it is a pest, it would have made it through on a dipped frag plug.