The green banner ad at forum header – it comes together from tiny pieces

Hey DP. There is one particular ad on the head of the site that sometimes , from the corner of my eye, I see it first in little sprinkles or bits that all jump together into shaping the banner. What kind of banner/script makes this happen?
Or is this just my eyeballs/browser misbehaving? Not semrush ads, but a different one. (I think)