I have had this 75 gallon tank over a year, but just found this... But, what is it??? It opens and closes when startled, but I can't get around the back of the tank for a better shot..
now after researching and asking around i come to think hes just a bivalve creature or muscle no big deal he'll go away or stick around for a long time just a filter creature
usually means it came from your live rock and you've managed to recreate its environment good job