Is this a formerly-paid-for game which has now gone permanently free-to-play? If so then in future could you please save this sort of thing for [Forever-Free Friday]
Steam isn't showing it as on a 100%-off sale and is labeling it as "free to play", so I think it's probably permanent. Don't worry about this one now (up to you if you want to edit it), but in future just save games like this for Fridays.