Its either pigment change based on diet or infection but see either being the answer .
They do require diet other than the belief they only need algae. Water quality also of importance. The trickiest part about keeping a blenny is making sure it gets enough to eat. You can tell this visually by looking at the abdomen, which should be a healthy rounded shape. A blenny with a pinched midriff isn’t finding enough food.
Although a blenny needs plenty of algae to survive, a common misconception is that it can survive on algae alone. Since blennies frequent shallow tropical waters, they find their sanctuary within the coral reefs. They circle areas with coral branches and sponges, scraping their way along the diverse sea bottom. Their feeding techniques are to pound and gnaw the coral, and as a result, they ingest detritus, or waste material primarily, from the coral. This includes bits of coral skeleton, sand, gravel, and other organic matter, such as fish eggs and tiny crustaceans.
Since a blenny in the wild does not subsist on solely algae, it stands to reason that their captive diet must be supplemented. If a blenny is not getting enough calcium carbonate in its diet, its condition will rapidly deteriorate. Therefore, it is important to offer an occasional selection of commercial algae-based wafers or pellets.
