Clownfish sick?

[bromion]

I am worried that one of my Ocellaris clownfish is sick. I've had a black & white Ocellaris pair for a year and a half. They have both been healthy and eating well throughout that time. Yesterday, I noticed one of them was moving its mouth a lot and pressing its head against the rocks. Atypical behavior for this fish. They host in a Xenia colony, and the fish was coming out less than usual, though when it did, I did not notice any other symptoms of a disease (no spots, mucous, torn fins, or discoloration). The fish also ate.

Is this an early sign of illness, or am I unnecessarily worried? What action should I take now, if any?

Thanks for any advice!