Need a little more info please. It's hard to help point to possibilities without being able to diagnose all of the factors.
How long has its mouth been open?
Anything coming out of the mouth?
What are you water parameters?
How long have you had it?
Is it moving around in the tank?
How old is your tank?
Do you feed it?
Let us know. Mine sometimes looks like this when its inflating and deflating as moving around but its not open for an extended period of time.
Just from looking at the rock in the background though I would guess this tank is extremely new. That's not always a good thing.