What you see: White film on grapes
What it is: “Bloom” produced by the grape as a natural protection
Eat or toss? Eat! It’s just part of the grape’s natural coating, nothing more.
I admit that there was a tiny part of me that used to eye the white film on grapes with suspicion, assuming it to be some very obvious pesticide residue. Of course, nothing could be further from the truth. The white film, called “bloom,” is something that you see on a number of soft fruits and protects the grape from things like moisture loss and decay. Produced by the grape itself, the bloom is mostly made of a waxy substance called oleanolic acid. Oleanolic acid occurs widely in food and might even have some disease-fighting properties.
California Table Grape Commission - Grape Terms
Waste Free Kitchen Handbook by Dana Gunders p. 151
Pharmacology of oleanolic acid and ursolic acid
The white stuff on grapes? Why, that's just a little self-preservation.