Looks like bacterial bloom
How old is your tank and what are your parameters?
Have you used carbon dosing at all. This includes pellet food. If you are this could be a bi product of an imbalance
If your tank is relatively new then it’s nothing serious amd will go of it’s own accord providing your readings are good. No excessive Phosphate and your magnesium is within range. Keeps your KH in check
Providing none of the above have changed and your tank is quite new, can take months for it to settle down as you are still putting the building blocks of life in place. Then I wouldn’t worry to much. Yes it doesn’t look great but Rome wasn’t built in a day. Do as you are at the moment take off excess and keep your parameters stable it will clear u
Sponge it could be - but as mentioned bit early and yes welcome to ReefBase