In my opinion, the people in the relationship are poly, and the relationship can be described as de-facto mono, but really it isn't monogamous in the way most people would understand it. So yeah, calling it poly is fine, even if it might confuse people if they ask about your partners and you say you only have one.
I think whether you say your relationship is mono or poly you probably need to specify ("mono, but open to poly" or "poly, but without other partners" or something).
If you're an individual, saying you're poly is only talking about your orientation and works fine. You don't have any partners, that doesn't make you mono, or celibate, or anything like that.