YieldStar is basically an outsourced price fixing scheme. The app assembles data on average rents in an area from across all the different landlords who use the app, and then recommends the "optimum" price for rentals in the area. "Optimum" of course means "The highest rent this market will bear."
The users will, of course, claim that this isn't collusion because they're not directly communicating with each other, and it's not price fixing because "they're just following the recommendations of the app." The whole thing is just criminal conspiracy outsourced to a third party, and the reality is that it's pretty much impossible to stop stuff like this from happening somehow. This is exactly why aggressive rent controls are essential to protecting renters.