What exactly is happening? Just hair algae or something else along with it?

You probably won't get a phosphate reading, no matter how fancy or expensive your test it. The phosphates are utilized, so you can tell what's there by your algae growth.

Phosphates, like other things, are needed in moderation. When we over feed processed foods or use low quality water, it exceeds the moderate amount and causes nuisance algae.

If you changed the source of the water you're using, you threw off a balance. Even if it's good water, it's still different.

When I used tap water, people freaked out. ("It's full of phosphates and silicates and nitrates! Oh my!") My tank still never had any nuisance algae. My macro grew like crazy and so did my sponges. When I switched to the water mill, I had some changes in the tank I wasn't happy with even though the water was better.