Puffer belly rash?

One of my puffers belly gets grey patches on it from time to time. This did not
happen for the first year I had them except when they were first brought home.
This particular puffer has allowed its teeth to grow too long and now it is
buck toothed. It still eats flaked food with relish but it does not like snails
like the other two, so it does not wear the teeth down as much as the other
two. Any way the grey patches often dissappear nearly instantly when I feed
them and they are all happy. Any one know what is up with the grey "rash?"