I have seen this but have not been able to figure out what exactly causes it. I have not seen a yellow clown goby in any LFS around here that does not have this condition. I would be interested to find out what exactly this is. It's not like ich. I have not seen anything similar on other fish.
Can you post parameters?
How long have you had the gobies and if these are recent additions, how did you acclimate?
If they can be caught I would consider putting them in QT, no copper, feed them well with vitamin and garlic supplements added to the food and maybe treat with melafix.
How long