This is usually because of poor water quality. You might need to get your tank cycled if you haven't already or you might have a mini-cycle on your hands. Did you already test your parameters? What are your ppm of ammonia, nitrite and nitrate? And fine is not a number, so if your LFS tests your water for you and tells you it's 'fine', slap the numbers out of them.
For the record, plecos do breathe air, as most catfish do. The sailfin pleco even breeds (or aestevates, I forget) above the water line in muddy burrows. But in a healthy aquarium, a pleco won't usually seek atmospheric air.