Remove sticked Webcam Cover from MacBook Pro display
The damage is already done by whatever adhesive was applied. Apple only recommends one solvent, pure water, so I would use a plastic spudger to gently dislodge the slide and then clean as best you can with water. 

If you need a solvent, flip the screen upside down so any solvent drips over the normal top, now bottom onto the aluminum as opposed to dripping down the glass. 

Chances are, the anti reflective coating will only be weakened or damaged in that top area or not at all. Damage to that layer there doesn’t spread, it’s only where a solvent or physical abrasion is present, so carefully control abrasion and solvents and your screen area should be safe. 

Taking it to a shop that has tools and experience with solvents might be well worth the trip. Take them a pie or cookies, they will love the challenge and you’ll be in good hands if you’re not sure of your skills.  

My hunch - you’ll get it off and whatever damage happened will be negligible and no issue. Please post a follow up picture once you’re done or answer how you solved this!