Thats what it did Jeff! Its a circle about the size of a quarter and its yellow grey as you mentioned. Im pretty sure its p'd off about the Big sailfin tang. This morning when I went down stairs it was still hiding, but usually its out swimming around with the foxface. It came out a few minutes later with that funny spot and its top fin at full mast, lol. Im pretty sure hes not happy with the new tang; although the new tang is loving the tank,lol.