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Recently, my bro's corydoras has developed a small white bump just above it's mouth. It's a very irregular looking bump and at one point it appeared to swell. It's not white, per say, but the tissue is lighter than the surrounding tissue which is the typical bronze color associated with bronze corys.

I can't comment on the exact quality of the water, but it appears to be well filtered, cycled, and I'll bet the hardness and pH are fairly high because that's the way water comes from the tap and there are no live plants. Also, the tank mates are passive towards each other and include a spotted cory of some sort, a white cloud and a ghost shrimp.

I thought maybe it could be a fungal infection since the fish will sometimes swim to the top for air and expose it's mouth. I would include a picture, but my digital camera has very poor macro abilities--so maybe later.

Any thoughts?

Check the gravel. Does it have sharp edges... if so, it may have been the cause of injuring the cory's mouth.

This sounds a little like MouthRot/Mouth Fungus. If so, it's a bacterial infection

Cottonwool fungus could also be a possibilty esp when there are wounds.