[bettababy]

How big is the trigger and how old is it? They will typically go through some color change when/as they mature. When you say it's rubbing it's mouth against the rock, is it causing tissue damage around the mouth area or is it possibly sharpening it's teeth on the rock? A trigger needs to have something to "chew" on to keep the teeth sharp and in good condition. Their natural diet usually does this for them in the wild, as they are big eaters of shellfishes and snails. I mention this only because I didn't see any mention of injury concerns, and the fish seems to have chosen a favorite "hard" rock to use. It would really help a lot for everyone to see a picture of the fish and the tank. If you don't have a camera, is it possible to borrow one or ask a friend to take the picture and email it to you?
This sort of problem is very difficult to sort out without being able to see what is going on. The more you can offer us for info, the easier and faster we can help you.

 

[bettababy]

I just noticed, none of us has warned you yet that your trigger is going to outgrow that 55 gallon tank. It's probably a good idea for you to start planning a larger tank, cuz once these guys start growing, it goes fast, and they get really big. A safe tank size for 1 trigger is 125 gallons.