Which is right: a backup plan, a back-up plan or a back up plan? How about a cutoff date, a cut-off date or a cut off date? A takeout menu, a take-out menu or a take out menu?
The answer: There is no answer. Technically, you could choose any of these forms and be correct. That's not to say that they're all equally good, obviously. But they're equally legitimate due to a weird little quirk of the language that occurs at the intersection of hyphenation rules and modifier rules.
Here's the basic idea.
Adjectives, which are a type of modifier, describe nouns: a large store, a nice day, an efficient employee. But in English we can use nouns the same way — to modify other nouns: a paint store, a vacation day, a government employee. We call this an "attributive" use, meaning a noun like "paint," "vacation" or "government" is attributing to qualities to another noun. In other words, it's a noun working as an adjective, which is standard and correct in English.