Can we see a full tank shot? And a sump shot?
For my tanks, I run my lights on a 12 hour schedule. 8 hours of high light, with a couple before and after actinics. Everyone's different, but I try to give them what they'd get in the ocean. Many people keep them alive with only 8 hours of light. They may not thrive on that much, too many factors. I'm not familiar with the AIO set up you have, so I'm not sure how that may impact your temps. As for water changes, again, if it was mine, I would do 20 gallons at a time, maybe a day or two apart for the first week or so, while that carpet is adjusting (keep in mind, some anemone's need MONTHS to FULLY acclimate). I would use a siphon to remove any unwanted growth, the dark stuff on your rocks is probably cyno, and should come off pretty easy with a water change. IMO, that kind of starts a snow ball effect. Cyno chokes out the rock, smothers good stuff inside the rock, it dies, releasing more crud, on and on it goes.....
The stuff growing on your rocks, does it seem to get thicker, or grow more, when the light's been on a while? Does it look thinner or less in the morning before the lights come on? Altering your lights that way, won't really help NOT growing algae. Limiting the food source WILL help reduce algae ( limiting phosphates, nitrates, feeding sparingly). Well water (I'm on a well too, tons of rust) may be part to blame too. If so, water changes with unfiltered water may be useless in helping.