Green clumps of (hair?) Algae

[MitchM]

Yes, didn't you know that foxface fish are photosynthetic...? <jk>

Poor lighting can contribute to nusiance algae growth, and you should test your source water every now and then to be sure that you aren't adding unwanted chemicals into your tank. (Which could also be in some water "conditioners") (You didn't mention which one you were using, BTW)
That's why so many reefkeepers use RO/DI units.
Excellent water quality is extremely important.

Your tank PO4 measurement could read zero, and at the same time, you could be having crazy nusiance algae growth. That's because the algae is consuming PO4 as it grows, so it may not show up on a water test. But testing is always a good idea. You'll never know otherwise.

Online sources like this one are great, but most of the advice is anecdotal, and you should really have some quality reference books by qualified authors.
I like to use the web to get me pointed in the right direction, then do some serious research to back it up.

Mitch