Someone correct me if I'm wrong, but the video sure looks like brown jelly to me.
Yes. It does
Most likely from poor water quality
I think op should take the chemipure out
Do 20 percent water changes once a week
Make sure to blow rocks and siphon 1/4 of sand bed every week
Try that for a month or 2 and see if that works
Also I prefer liquid gfo remover. To me it’s easier to dose liquid then gfo. Gfo strips the water too fast imo