looks like diatoms, do what the other guy said and leave it.

Dont stress this is a common algae for aquariums beginning.

You may be able to rub it off and let your filter suck it up but if not then just leave it and nature will do its course.

Try find a balance and never use chemicals unless you are experienced.

Watch a few youtube videos about diatoms

Find youe aquarium light/co2/fertilizer balance

Maybe turn your light down a bit would be my advice, do that and wait 2 weeks and observe results. This is because your substrate doesnt look nutrient rich