It is one of your most dreaded fears, and now it's here before your very eyes:"illegal program has been discovered". Not only can it be lacking in grammatical sophistication -- periods are important, men -- but Maple M Mesos disturbs you from accessing MapleStory M.
Why are you able to access the game absolutely unimpeded, simply to have this warning pop up today, of all times? The first point to realize is that the program detection process is not ideal; it will miss things upon launch sometimes, and grab it later on.
The most obvious fix, obviously, is to uninstall some cheat programs you may have on your device. Anything that's made with the goal of hacking or altering the game should be uninstalled immediately. Otherwise, you face the risk of your account being banned permanently. Note that you don't have to be running these programs throughout gameplay for them to be detected; when the scans detect itis the only option.
The prohibited app message is also known to trigger in phones that are frozen or jailbroken. If your telephone is second hand and you are unaware of its own background, this may possibly be the case, as well.
As a final fix, consider reinstalling Vedio About MaplestoryM Mesos and relaunching it with no other programs running, even ones that are entirely irrelevant to hacking. You may have an innocuous app that's somehow triggering the warning.