Uhm I was looking up algae the other day and found something like this, it's called algae bloom.
This is kind of a pain to get rid of because every time you do a water change they will usually just grow back immediately. I read that the best thing to do Is cut off all light to the tank for a few days and do lots of water changes to stop the algae from growing.
I'm not a pro, so you should probably look it up some more.
I don't think a
uv is nessicary but it will help out your fishy's later on :p