Metalslime was a running gag where a grayish-green metal... slime... thing would attack characters' Spleens, accompanied by the scream "AAAAH MY SPLEEN!"
Narz wanted to join DYOS V, but he couldn't think of a character to use. Stylesjl suggested an Empire Earth character.
Narz also wanted Metalslime to be included in the comics, so Stylesjl spray painted a grey middle with green spray on the outside which yelled "Gleeeaaarg! I am a crappy drawing"
Thus, Metalslime was born.
The Styles brothers use Metalslime a lot in many situations.
It was funny the first 5 times it was used, but got overused to the point that the pain of seeing the joke used YET AGAIN was greater than the pain of actually getting your spleen eaten.