Looks like it's coralline algae or calcium deposits that you have on the filter, with fluffy green algae around it? Phone pics are lame haha. If it's coralline it will feel hard, usually occurs in saltwater tanks but freshwater is a possibility. When I get in my hard water tanks occasionally, I take the filter out and get plain vinegar. Soak some paper towels in the vinegar and then let it sit on the filter for like half an hour. After that, get an old toothbrush or a scrubby sponge that has never had soap used with it and start scrubbing. Usually that gets it pretty clean, then just rinse it and let it dry. The vinegar won't hurt the fish after it's rinsed and dried. If not a razor as stated above should work, I would do that to the glass if there is any on it. The fluffy green stuff comes off super easily if it bothers you that much but I would leave some for the fish to munch, most like that type. I would leave it on any rocks too, IMO looks more natural that way.
Maybe a blow dryer can help loosen the glue up on the wallpaper.
Also perhaps a few live plants and more water changes will prevent that algae from returning. You should also try to put the light on a timer for around 8 hours or move the tank where natural light can't hit it. Seems like you have a lot haha.
PS... is your tank saltwater? The star thing in the pic looks a lot like a starfish haha.
