Fortunately, I don’t think there’s anything to be too concerned about here
In a general sense, “sneaking” refers to the in-game control where you crouch down (commonly done by pressing “Shift” on a keyboard or by double-tapping the button in the center of the D-pad [directional] controls on mobile).
By “sneaking” in this case she might be referring to when someone taunts another player in Minecraft by repeatedly sneaking and un-sneaking, (though it can also be viewed as a friendly greeting which depends on the context)
Unfortunately, it seems that some people misuse the word toxic; maybe they don’t fully understand the meaning of the word; (this person might be a kid)