Intex 22' x 52" Round Above Ground Pool - 2500 GPH Cartridge Filter
10,000 gallons - PF=12

A pH of 6.6 isn't unsafe . . . but the problem is, it may not be "6.6", either. Probably, that's just as low as your kit or strip goes, so an actual "6.6" or an actual "4.6" would still READ 6.6!

Don't swim till your pH is ABOVE the lowest value your kit can detect.

Are the stains gone? What is your stabilizer level? What is your calcium level?

(Thanks for the subscription!)