Pleco acting weird...

My common pleco usually just swims around, eats my goldfish's food, and gives them a hard time, but for like the last three days, all he's done is go to the back wall and just slide down and come up to the surface and slide back down, etc. Well, at first, I thought it was really cute so I wasn't too worried. Rarely is he ever in his right mind since he's started to do that, and algae is going crazy, and my pleco needs to get in his right mind and start sucking, and I'm pretty worried about him, tbh. Just need to know why he acts this way, and why he does this. Oh, forgot to mention, whenever he did finally suck last night for about 30 minutes, he never pooped. Attached (I hope) is a video of what my pleco has been doing.