It is often up to interpretation. There was a big hubbub a year or two ago about someone who went dressed as a storm trooper. One night they had no problems letting them into the park for the party. The other night they were stopped on Main Street and asked to leave. Both times they had security check over the mask, make sure it wasn't an obstruction, etc. But they were later told is ends up being the decision of the manager of the park that evening. And the second night the manager said that mask was not allowed.
However, what you are describing should be ok. Make sure that it has a decent sized eye hole and you are good. As a fall back, bring a white thing of face paint, and just paint one on if its an issue. But I imagine that will be ok.