I have a very dull light yellow splotch on the back inside wall of my tank, and it has moved every time I check on it. At first it looked like a flower because of a line that went down into the sand, but since it has moved. I love my tank and watch it every chance I get. It is very relaxing and new things happen every day. I have a couple bristol worms, but I haven't seen them lately. I don't want anything to upset my tank, and hope anyone can help.