Been at this for many years. I've not seen something like this. It is slimy. Easily comes off the rock when rubbed. Only grows on the sides facing the lights. Gray in color. Has a few bubbles here and there. I can remove it with a siphon but it grows right back. Never seems to get larger (longer) than what you can see. Its throughout the tank but more in areas with lower flow. I think its some sort of biofilm that is bacterial related. My tank is extremely low nutrients and I believe I may be adding too much "good" bacteria. I've cut back on dosing both microbacter7 and Dr. Tims Waste Away to see if anything changes. It does irritate coral. Thoughts?