Yellow clown gobies are fun fish. However, more so than any fish I have ever owned, they are VERY sensitive to changes in water parameters. I seriously doubt I will ever buy another, even though I think they are really cool.
When I was having issues... I was discussing these critters on Talking Reef.com and a user (JustDavidP) gave me a really great response that I wanted to share with you now.
Check out this FAQ and other related Gobioid at Wet Web Media:
http://www.wetwebmedia.com/gobiodonfaqs.htm
In particular, read the thread titled: Yellow Clown Goby with "parasites"
It should also be noted that Yellow Clown Gobies, or Gobioids in general, do NOT tolerate degrading water conditions. They are a scaled fish, but do not fare well with swings of their environment. It has been noted that they create a mucous much like mandarins do, to assist in this issue of irritants and reaction to said irritants (much like a scaleless fish). The slightest rise in nitrate, nitrite, amonia etc. causes them to stress.
They are also a bugger to kept properly fed....
The bottom line...they are easily stressed fish. Soo.. with that said, stressed fish are succeptible to disease or infestation. This is why I firmly believe that MOST tanks contain ich and that it only becomes a problem when you buy that "special fish" that is a swimming nerve cell.. IE.. YCBs, Tangs.. and BANG.. the new fish has ich, while the others do not.
Best of luck! I wish I knew more about them. Personally, after seeing a pair strip an acro (breeding response) in a friend's system, I kept away from them.