I'm looking at the fish and I don't see anything specifically wrong with it. The tail doesn't look damaged or infected. If the white is due to an infection, it will likely keep going up the length of the fish and kill it. However, it's very much possible this color is normal. You may not have the fish species you think because several fish in the trade go by tribal blenny. It's also possible you have a juvenile color form and it will change to the adult color form in time.
I'd say that if the fish is eating normally, swimming normally, and shows no sigh of disease, just leave it alone. Above all don't panic and start treating it with anything, unless you see a specific problem.
I have a tribal Blenny. A couple months ago he looked as if he was starving. Very skinny and face area was sunken in. I was feeding a mix of brine and mysis. I added in tropical flakes and red algae sheets. I also treated with “prazipro”. The blenny has gained weight back and looks healthy weight wise however his tail has turned a white color. It seems to be moving up the tail very slowly. It doesn’t appear that the fin is damaged in anyway. I have researched and can’t find anything on this. I’m pretty sure it’s not normal though as he didn’t look this way before. If anyone has any information or advice it would be greatly appreciated. If you need more info let me know I will post it. Thank you in advance
to RS !