Ah..used to have them before. Some sort of an evil copepod/saltwater wood louse. It's actually ###### blood out of those fishes. DO NOT LET THESE PARASITES BREED ! KILL THEM ALL !
You could try to scrape it off if you want to. I did it once. But it seems to stress the fish more when you net the fish and have the copepod manually removed.
Anyway they'll usually come out in the dark. Once you spot them on the sandbed, net it immediately and KILL IT !