This highlights the foolishness of public social media accounts for young footballers. Stuff like this makes the job even harder than it already is for a manager, and can undermine a manager unnecessarily. It could be just an in-house joke that Warnock already knows about, but we know probably it isn't.
If it isn't, then even if just meant as a joke by the players, it's easily read by the outside world as a lack of professionalism and respect. The club should get a handle on this kind of thing. The money players are paid, it's not unreasonable to place contractual obligations on them regarding their public interactions, it certainly applies in many other professions, most of which aren't as well paid.