I wonder if it is hair algae, then.
Any chance it could be a beneficial macro algae? Can you describe it, or post a pic?
I'm not an R/O expert, but I wonder if ammonia added to the water system could cause a carbon block in a filter to leach PO4...
(stab in the dark..lol..)
Post your water tests when you get them, too. (both tank water and source water)
Mitch