Other causes of algae

That's is the question.....Besides N03 and P04 what else feeds it? Not having a crazy outbreaks but still have plenty of algea, GHA, calerpia, bubble, andsome new real short red turf looking stuff, all this with no readable levels of P04 and N03. Using a Hanna checkers for Phosphate and Salfert plus a ATI for N03. What else could I be testing for? This is not plague proportion or anything but still annoying. All other levels are great, ph-8.3, alk-8.5, Salinity-1.026, calcium-480, magnesium-1400. Could it just be 14 year old live rock is feeding the stuff? Thinking of getting rid of the 14 year old crushed coral and going BB for a little while but hate to kill all the little guys living in it. Only have 1 inch or less. And it gets vacuumed every 2 weeks with my WC.

So, any thought here? Is there more then just nitrates and phosphates that feed algae???