Bro, your problem may be similar to mine. A few months after I started dosing A,B,C+M, my tank is covered with this purple-reddish algae,see attached. The PO4 is less than 0.03 and NO3 is less than 1. Solved the problem by using UltraLife red slime stain remover. I only used one dose and only 80% of what is stated in the instruction. After dosing, thin layers almost disappear on the same day, but those thick thick layers took a few days to clear.
I am still dosing A,B,C+M weekly but now only use 10% of recommended dosage.
