Having brain surgery tuesday morning, preop tomorrow. will i get denied because i have a very mild case of thrush?

Maybe: Make sure you make your surgeon aware if this new problem as soon as possible. If you can call their office monday, they will have more time to determine if it is safe to proceed. Naturally, it also depends on the reason for the planned surgery whether or not it should proceed as planned. Mild thrush can usually be effectively treated with medications.