Probably dinoflagellate algae guessing by the bubbles...
Could be diatoms or
cyano also.
Turbo snails if it is dinos I believe..
Also 20 or even 10 is pretty high
PO4 for reef tanks .. Ideal is .03-.07 with many liking .05 depending what corals they keep...
GFO will be your fastest method to remove
PO4 safely.