At 5 months, hair algae is part of the maturation phase. just like tha diatom bloom earlier on. That's around the time I developed hair algae. Keep up with 10-20% weekly water changes, make sure your source is zero phosphates. If you have no room for a phosphate reactor, try to run passively or in a bag or a hang on sump filter like those used for freshwater tanks.
Even the Hanna photometer will read zero if you have hair algae. The point of the drip is that the phosphate precipitates out before the algae can use it.
I tried it all: algae fix marine, turbo snails, emeralds, sea hare (bad idea), phosphate reactor, manual removal, 3 day lights out, wet skimming. In the end perhaps it just ran it's course. Good luck!