All of my blue reef chromis look like this (6 of them). Only other fish in tank is a yellow tang, black ice clown and 3 green chromis. Water parameters are normal and all other fish are doing great.
Blue reef chromis are impossible to keep IMO. I don't know anyone that has success with them. I don't know what causes it and I don't know that anyone knows.
Look at Vibrosis, although there is not a lot of information. Basically it's a bacteria that causes skin ulcers and seems to be a common infection with Chromis.