The tail injury can regenerate as long as it's not too deep, and relatively quickly.
The spot is fuzzy on a small fish and hard to see clearly. If you just taking possession of it in the last day, This could be mechanical damage doesn't look like a parasite that I am familiar with. Keep in mind that the supply lines are pretty nasty right now. Fish are waiting for transport a lot longer in holding facilities. If you just brought it home from your LFS, it's had a trip for the last 6 weeks or so. Good water quality and nutrition should be top priority.