Probably one of the weirdest pop ups I've ever seen.
I have MyFitness Pal installed, although I seldom use it anymore, and I tried to get the pop-up to happen on my device with no success. However, from what I noticed, the app seems to use ads to warrant the "free" version. It's very possible that the quality control has dipped and they're allowing "sketchy" ads to filter through and I can also assume "sketchy" ads pay more. This is all just speculation but a quick search proves you're not the only one with the issue.
I wouldn't worry too much if you don't mind ignoring them, although personally, I wouldn't continue to use an app that contains questionable adware.
Edited by C0bra, 29 January 2018 - 03:30 PM.