Lympho or pehaps a bacterial infection setting in. No cure for lymphocystis. Fish needs pristine water quality and as close as possible its natural foods. Best way is something like LRS or mixing up a fish "smoothie" made from a variety of frozen cubes. The greater the variety the better + vitamins like Selcon, etc.
If bacterial it needs wide spectrum antibiotics in a QT: kanaplex, Furan2, NFG, Sulfaplex (triple sulfa), etc.
Doubt the fish will get either of the above treatments at Petco. Maybe someone you know can rescue the fish.