the CYA deteriates over time, there is a bacteria that "eats" it, and produces tons of ammonia...
You really need a test for FC and CC; you probably have lots of CC which will go away with time (FC and UV get rid of it...)
with your chlorine at 5, we don't know from strips if it is 5 or 5+ and how much is CC and whether you lose any (if it is still 5+, we don't know how much +) so can't tell about algae and such.
take a water sample to pool store, if you can, and get a full test including metals- Cu can sometimes be green just like algae