This is a clingfish. They are a very cool species of oyster reef fish known to eat algae and the like. I know a place where I can find a practically unlimited source of them. They are super underrated and can live in brackish and saltwater. They even eat feeder guppies, somewhat surprisingly. Generally similar to otocinclus, hillstream loach, and small plecos. Very cool. I might even be willing to sell some one day but not any time super soon. Just something I want to share. I had 2 in my tank-- but I think one might have died, unfortunately.