The flap part of the seal on the top of my main slide did the same. I tried gorilla tape and even eterna bond rubber roof tape, but there is just too much pull and tug when the slides go in and out for tape to hold. I ordered a new seal piece and replace the whole run.
My problem that caused the whole deal was the white glue they bond the unit together with. It oozed out and the seal stuck to it when the slide was retracted. When I opened it, it ripped the flap part. I removed the glue from various places around my slides and have not had the issue repeat. I see a bit of it on your seal as well.