Parasite / Cyst / Injury

[SarahBravo]

This is the smallest of my Lake Inle Rasboras (thanks Wolf). He has a white cylindrical 'lump' at the base of his tail. It is definitely not a mark on his skin as you can see it from both sides, and the outline of his tail curves around it. Does anyone have any ideas what it could be? Should I put him in quarantine in case 'alien' is about to burst out of his tail?

Any thoughts/advice very welcome.

Sarah