Spraying with neem oil like that has probably totally clogged up the pores of the plant--that stuff is oil after all. Do not treat plants for things they do not have. If you are worried about mites, get a preventative like SNS 209. Besides, I have never actually seen or heard of neem oil actually doing any good against spider mites. Those neem oil guys must have some great sales reps--so many new growers buy it and just use it randomly without really understanding what it does (and doesn't) do. Stuff is nasty. I also hate it.
DO NOT DO ANYTHING (except quit using the neem oil) UNTIL YOU KNOW WHAT IS WRONG WITH YOUR PLANT. It sounds like mildew to me, but I live in a place that has low humidity and I never have had it. Do you have an extraction fan in your space?