I seem to notice it at random parts of the day...but i'm not sure ) I've noticed it for a while but after I saw it again this morning after turning on the lights i thought I'd ask around
if its a little white layer it means he is doing the equivalent of shedding. They *shed* to rid themselves of parasites. They also do it sometimes when there are no parasites.
If it is happening at all times of the day, it almost sounds like something may be stressing him. Do you have a dog that may be barking or kids running past the tank, ect... Keep an eye open for external stressors.
The de-coloration appears just in one little spot on top of his yead, I don't know that it would be some sort of shedding in the same little spot. After he ate yesterday he seemed to get his color back. Maybe I'm just not quite feeding him enough (I do tend to be on the conservative side) )
As far as external stress factor, there shouldn't be any. I live by myself...Unless my tv or radio are too loud (or my neighbors, i live in an apt.). No dogs, cats, kids, midgets, etc...