Yeah its just diatoms, pain in the ass but its going to happen.
This appears due to high nutrient levels in your tank and a few things will trigger it more then others.
Im not going to ask your numbers, Im going to assume you test and you know where your at.
A fee questions that is directly related to this though.
Do you have a skimmer? This is because it would make sense that you have high nutrient levels if you dont have a skimmer and if you do have one then it might be to small for your system. If this is ok then move to the next reason.
Do you supplememt anything into your aquarium? This is because if you supplement to much then your going to overload your skimmer and system with more nutrients then your system calls for and bam you have diatoms. The main thing that will do this is dosing iodine. If you dose iodine and you start to see these diatoms then you are dosing to much because diatoms love iodine.
So thats about it.
Check your skimmer, watch your dosing.
Scrape off your glass then do a 10% WC and dont dose for a little while then do two more 5-10% in the same week then dose half what your dosing. Remember when doing large WC's to check pH, salinity and temp of your trasfer water so as to not shock your system. You should be good to go.
Just remember we all battle this at one time or another, it is all part of the experience.
Cheers!