u/Interesting_Pay_581

I just noticed this on the inside of the glass to my 80 litre today. It is around 5 cm or an inch in diameter and looks fuzzy and pale white at the edges, with transparent dots in the middle. It sits completely flat against the glass, and you can only really see it when light hits it from the right angle.

Anybody have any idea of what it could be, and if it's beneficial/harmful?

Thanks