Filament residue sticks to bed and wont come off
I would try a single edge razor blade at a low angle used as a scraper.  If you can't feel the blade catching in the residue,  it probably isn't an issue.
If you must get rid of it you don't have good choices of solvents.  Maybe you can burn it off by placing the class in an oven through a clean cycle.  With luck the hearing and cooling won't break the glass.
If you can't scrape it off with a sharp razor blade,  I would just let it be.