Can you get a better pic? if it's the blue thing, unless that's the light reflection. almost looks like a feather duster, but cant tell. I'm honestly just guessing here. I di t see anything but rock, bubbles, and cheato in water.
Still need a better pic. Rotate the cheato and then snap a pic. If its sessile it maybe cyano, bacteria etc. Get the phone closer, dont just zoom in please.
Definitely need a better picture to get a more accurate ID but I would think you are probably looking at cyanobacteria. Do you have any cyano, slime, or nuisance algae building up in your display tank?