It may something similar to an abscess in mammals. Since it is growing so quickly, there is fair chance it may soon burst. If it does and whitish stuff comes out, the fish will probably be fine once it's all drained, if the wound does not get infected.
Don't try to hasten the process. If it does not burst, and grows fast, there's always a chance it is a tumour, in which case the fish will probably die. I hope not.. the abscess is much more survivable.
Keep the water very, very clean. If you are using these fish to cycle a tank, there's a fair chance the fish will die because it's an added stress, being unwell and in less than cycled clean water. If possible, I'd return it to the store and ask for a replacement, as it must have had this when you got it.