weird stuff on scopas tang

[fewtrain]

hello all this is my first posting on this forum ive had my tank up for about 1 1/2 months now with no problems i purchased a scopas tang from perfect pets in richmond, va that i beleive brought a weird disease with it ive showed it to a few people at local shops but it is still unknown what it is it doesnt look quite like ick and it comes and goes every few days the tang has been in the tank with a six line and a goby and a few shrimp and some coral with live rock for about three weeks his symptoms appear as looking kind of dusty with hazy eyes and he gets really dark sometimes i know they are prone to changing colors but not like this i wouldnt think none of my other fish or inverts have been affected but i did have a percula clown that i had for nearly 8 months die all of a sudden but he never showed any symptoms of this disease the tangs breathing is not heavy he is eating fine and moves around constantly he just looks like crap sometimes ive had my water tested it was fine then i went ahead and did a 15 gallon water change and deep clean but still the tang looks bad ive tested my water nearly everyday since the change and still perfect so im puzzled as to what to do next because i would like to stay away from chemicals if possible tonight i plan on changing the filter pads and possibly another small water change but if anyone can help me with this please do