Green spotted puffer belly periodically darkens

One of my green spotted leopard puffers white belly turns grayish or nearly
black from time to time. I think it is some sort of rash. It has nothing to do
with feeding.But it has me woried because it is showing signs of distress.
curling it's tail around its bodie and resting on the floor in well lit
conditions. It is normal for them to rest on the floor in the dark but not with
the tail curled. That is a behavior seen in showing passivity to a dominant
fish or when scared. The other two have occasionally greyed like that but
always returned to normal. This one briefly returned to normal but resumed the
grey coloration after a few minutes.