It may be a unique tag used by an individual or a bot in the comment sections of news sites or blogs. A Custom "MUGEN" Mod:

To maximize your Mugen H Slime Plus experience, here are some expert tips and tricks:

Visually, Mugen-H-Slime-Plus is simple but effective. The slime itself is usually rendered in a translucent, glossy style that stands out against almost any stage background.