I was diagnosed with mono 3 days ago. is it normal for it form into a white/green/black sheet that covers my whole tonsil? if so when will it go away?

Mono: Yes your tonsillar tissue can become necrotic and give you a greenish black exsudate.

Can happen: It's very possible with mono to have "exudative tonsillitis" with gross-looking material covering the tonsils. In the past, before vaccinations, we'd worry about diphtheria (which is very dangerous) but it's probable you were vaccinated against it-- if not you may need a recheck by your doctor. Gargle with salt water and hopefully it will improve in the weeks to come. If you're worse, see md.

Common: This is fairly common occurrence in mono but your situation sounds a little more extensive. Treatment is seldom necessary unless symptoms are severe in which case there are a number of local solutions that can be used as a gargle or mouthwash.. Check with your doctor or dentist. I would have your doctor or dentist look at your tonsils just to make sure you aren't experiencing some other condition.