I would say GHA, looks similar to what I had..I can tell you no matter what type of algae your phosphates are probably higher than that reading as the algae is consuming and feeding off of the phosphates. I tried a number of things, manually remove lowered phosphates, etc but in the end vibrant took care of it
It's either GHA or bryopsis. When it's thin, bryopsis tends to be pretty easy to tell apart from generic GHA, but when it gets thick like what you have it can look pretty similar.
Either way, you're on the right track to getting rid of it. Keep up with RO/DI, do regular water changes, and manually remove as much of it as you can. If it's GHA, then several species of snail or hermit will eat it, though perhaps not when it's that long. If it's bryopsis, almost nothing will eat it.
I would guess the zero nitrates is caused by the algae absorbing anything before you can read it.
Green Hair Algae can generally be pulled up in clumps.
Some other things can just be siphoned off and may have bubbles (or grow in/on Green Hair Algae).
Personally I would try to start with manually removing it and focussing on removing the algae from around the coral first. Monitor nitrates and phosphates closely.
From here I would expect to find that your switch to RODI has solved your issues or you need to find a way to reduce phosphate and/or nitrate (increase export or reduce inputs).
At this point you may need to come back for more advise.
Also if you pull some algae out you can probably get a clearer picture for a better id.