I can't see how to do it from the mobile app, but I have an option to 'unmark this discussion as a question' when accessing on the browser. When you go back to this discussion on your computer, do you see that? I'm unmarking as a question now - so you may see 'mark as a question', unless it is available to the space owner only.
Screenshot from Android? The iPhone one lets me know instantly about new posts in discussions I've participated in, very handy.