I have "safe" MA from Ace Hardware. Is this OK to use for borating?
I don't know what that is, do you? I don't put anything unknown into my family's pool.

Fresh bleach or not, to shock a pool with 60ppm CYA the FC must be kept above 20 until it loses less than 1ppm FC overnight and CC <= 0.5ppm.

Mustard algae is often greenish yellow to brownish yellow and it seems to prefer the shaded side of the pool and aggregate in seams and small pits in floor. It often disperses easily with a brush or even a wipe of a hand. Mustard algae takes much higher FC levels to kill.

Do you think maybe it's just dirt that's too fine for your filter or never gets in the filter? Did the trouble start after a period of heavy usage - July 4 pool party?