ok looking at the last post you also have an anemone in there. Saltwater aquariums need a lot of research and you should always do research before adding any new inhabitants. I would suggest starting here
https://www.bulkreefsupply.com/cont...brs-tv/beginners-guide-to-saltwater-aquariums.
20 gallon hex is a really weird sized tank and hard to get equipment for. I would change out the coralife filter or mod it so that it uses regular filter pads which i would clean, at the very least, once a week. It also looks like there is a stock light/lid that came with the aquarium and if so is not gonna be able to keep the anemone thriving
it is algae in the water column. Both are algae blooms just different types