What would be appropriate to wear? ?

I'd go with the white cowl neck sweater dress, the tights, and the boots. Its simple but still elegant and in my opinion, would be pretty funny to see you get slimed. Or if you are going a bit further, you could do something a little more elegant, but it depends all on the atmosphere. If I was you, I'd be a little dramatic when the slime comes too. Btw, you're really pretty.