Ok what is the frequency and tempature of your bulbs. Also how many bulbs. I'm glad to see you started excell you are on the right track I would reduce your photoperiod to 6 hr and attemp to scrape off and remove what you can. You can also try to spot treat with excell it sounds like your issue may be lighting but we will be able to tell for sure when we get your water test back. I will be watching to help any way I can.
Also wattage of bulbs