Cathie

Try letting it get a bit drier between waterings and do keep that window open. Maybe the soil is so peaty that it's holding a lot of water in the pot. Hollies need all that acid, but if you repot, add a bunch of grit to the mix (1/3 - 1/2) so it drains faster, and don't let it sit in the drain water. Also, if you've been spraying it a lot, don't, because it'll 'feed' the mildew problem.