Post a picture of it but if it's just some plastic piece that looks like it's supposed to come off you better take it off. The other possibility is the "nubbies" are worn off because they tightened it so much to prevent it from leaking. You don't want it coming off by itself, suppose it comes off when you're on vacation for a week.
If there is no check valve to prevent backflow from the cold water you'll have a flooded house, a few minutes can do a a lot of damage depending on where the washing machine is located.
In any event I wouldn't trust something like that and I'd either hook it up with hot water or cap it correctly.