After checking the grow yesterday (2021-01-01), I noticed some of the yellowing I've seen in previous grows that served as early warning signs for an impending grow failure.
I checked my reservoir and found it was down to about two gallons left and the pH had shot up to 7.1. I then checked my AutoPot runoff in the tray and found it was at 7.2. I then took my Remedy CBD out and flushed it with 10 gallons of pH 5.8 tap water until the runoff measured about pH 5.8. The plant has been returned to the tray and the pictures you see this week are the following day and I'm continuing to measure at the tray more closely now as well.
I've continued to pull off some of the larger yellowing fan leaves but, other than that, I've kept this grow as simple as possible. The flowering stage is continuing as expected and the pictures show about how far along we've come.
Your best bet is to get beneficial organisms for roots. Someone mentioned hydroguard. I use this and have no algae issues. Having good organisms mean that bad organisms will be out-competed and won't be able to get a foothold.
Clean out your res very well again and start root bennies BEFORE algae sets in!