Ender 3 flexible magnetic bed

How on earth do you get rid of the filament residue off the plate, it’s quite smooth, so I can continue printing but marks and shapes of filament all over the plate!?

Good afternoon.
Could you please add some photos to your post so that I can better understand your question.
If I understood your question correctly, the best way to clean the bed is to leave it under warm water for 5 minutes and then scrape off the remaining plastic with a scraper. This works best for me.
If you have this problem, you may need to calibrate the Z-axis offset. But I can’t say for sure without a photo of your bed.

Second on the plastic scraper. They are sold under a zillion names and typically come with a 100 ‘blades’ and usually under 20$ Usual suspects, amazon, Ali, wish maybe… They work really well.

Was the print Petg? It should have glue stick applied first, not to get it to stick but as a resist. If it is a very thin print sometimes if it fails to release it can be very difficult to come off. Scrape, I never tried water you can use a putty knife or razor scraper too, be very careful they will chew the bed to death if you are not very careful and they also will do a number on you too given a chance.

Warm to hot water should dissolve PLA stuck to the flex bed. Can’t say what it will do to the surface if anything.

Shouldn’t do anything to the surface of the bed, PEI is an extremely resilient material!