"Is that yours or mine?"
Translation:Ĉu tio estas via aŭ mia?
Note that the message you're replying from is almost five years old (January 2016), and the "you" in the message made it to XP level 11 (something I did in Japanese in a few hours) and then moved on.
Rae.F already gave a good answer to this, but a few months later. For more detail, see the link in my reply to this message.
As Rae.F pointed out, "mi" is not an adjective root. More info on "verbing adjectives" and why to avoid it in general here: