Greyed-out calendar entries

I have a calendar entry that's "greyed out" as if it were an invitation, but I cannot find any way to either "accept" it, or to otherwise make it look "normal" on my calendar. It seems normal in every way--it's assigned to my "green" calendar, has all the required data, etc. It's not a big deal, but I'd like to understand what's causing it to be this way in case it happens in the future.