Scrape it. Before oak floor is laquered or oil stained, it goes through sanding steps, that will easily remove leftover traces of plaster if any.
Water from plaster won't permanently leave a stain on oak plank, cause there is not too much water in plaster.
Don’t know, mate used it when Mrs left a cotton pad on her dressing table. I thought it was ruined, couldn’t believe it when he brought it back. It’s was dark black with a hint of red in it. Come up like new.