periods of high humidity is not bad and will eventually go down on its own. i would keep the screen top covered. you'll notice soon that your going to need it to be covered as it will be impossible to retain proper humidity levels when its not. take your time and dont rush to change things just because humidity has been 80 something percent for a couple hours or even a day. it will go down with time on its own and eventually you will have periods of higher or lower relative humidity.

so did you determine if your snake has mites or not?