Mush is described as looking like a pink-skinned, scrawny goblin from a fantasy movie. He has thin hair, large eyes that are heavy-lidded with dark circles, and skinny limbs contrasted by a bulging pot-belly. 
Mush is a Changer, formerly classified as a Shifter, who can form an expanded amorphous or vaguely humanoid body out of loose debris. His body can divide, stretch, fork out and reconfigure itself into a mass of tendrils (not dissimilar from a mass of veins and arteries) that attach themselves to loose matter, such as dirt, compost, junk, sand or mud.
His body then brings this loose matter together around his body, allowing him to craft himself a harder and bigger body. He can also redistribute it across his body, so that if his arms are loaded with debris he can move some of it to cover his torso or head instead.
↑Weld took the folders and opened the one for the Wards, glanced through it to memorize the faces of his new team. Then he went to the next file, “Then the top priority as far as opposition goes is… the Archer’s Bridge Merchants? Superpowered drug dealers. A Shaker 2, Tinker 2/Mover 3 and a Shifter 4. These aren’t big numbers. Am I missing something?” - Excerpt from Sentinel 9.1
↑ 2.02.1The Merchants are a group that never took off. They don't have high aspirations, they just use their powers to make a risky business less risky, selling drugs. They're economically depressed types in an economically depressed area, making what money they can off people with very little money to spare. Bit-rate dealers (Skidmark), white trash (Squealer), and people down on their luck, trying to fix their situation in life and failing (Mush). - Wildbow on Spacebattles
↑Beside Skidmark, opposite Squealer, was Mush. He bore a resemblance to a particular pink skinned, scrawny goblin of a creature from those fantasy movies. His hair was so thin he might as well be hairless, his large eyes were heavy-lidded with dark circles beneath them, and his skinny limbs were contrasted by a bulging pot-belly. All of the worst features of an old man and a malnourished child thrown together. Except he was real; just an ugly, ill person. - Excerpt form Infestation 11.5
↑Mush – A member of the Merchants, capable of absorbing masses of loose material and debris to form a protective, shapeshifting shell of trash, dirt and other loose matter. - Cast (spoiler free)
↑Mush (Deceased) – Classified as a Shifter, Mush could form amorphous or vaguely humanoid bodies out of trash, mud, dirt and sand. Killed by the Nine.
- Cast (in depth)
↑His right arm had divided, stretched, forked out and reconfigured until it looked like a mass of reaching veins and arteries. He plunged it into one of the trash cans that Labyrinth was absorbing into the floor, and when he withdrew it, the tendrils had formed the connective tissue for an oversized hand crafted out of garbage. His other arm and much of his lower body had already gathered some garbage around it, letting him stand several feet taller than he had before. The skin of his head and body was peeling off into more tendrils, reaching for more trash and distributing some from his arms to his torso.
From what I could gather, he needed some kind of loose matter to form the body of his other self. Dirt, compost, trash, maybe even sand. Problem was, however fantastic his surroundings might have been for this five minutes ago, Labyrinth was screwing him over by cleaning things up, maybe inadvertently. One upper arm, his naked upper body and his nearly bald head were all exposed and vulnerable. - Excerpt from Infestation 11.7