Looks like diatoms to me. When my tank cycled it crept in over a 4-5 day time frame, covering everything... It was pretty heavy for 3-4 days, then I tossed in a CUC and is disappeared in 2 days (cuc was a mix of turbo snails and blue legged hermits). I am not sure if the CUC cleaned it up, or if it died off and disappeared on it's own and was just coincidental timing.