He is the Omnitrix's DNA sample of a Galvan from the planet Galvan Prime.
In the original series, He is voiced by Richard Steven Horvitz who also voiced Billy, Daggett the Beaver, Lugosi, Zim and Alpha's 5 and 7, Carlos Alazraqui in Race Against Time and Eric Bauza in Omniverse.
He is a Galvan, a gray frog-like alien that is five inches tall. The Galvans are a highly intelligent race responsible for many of the technologies shown in the series, including the Omnitrix, the Null Void Projector, the Null Void, their planet, Galvan Mark II (since the Highbreed destroyed their original homeworld, Galvan Prime, in the "Alien Force" two-parter "War of the Worlds"), and the Galvanic Mechamorphs (Upgrade's specie).
Ben first intentionally transformed into Grey Matter to search for a rare gold Sumo Slammer Trading Card in cereal boxes in "Washington B.C.".
Ben 10: Ultimate Alien
Grey Matter appears in the intro, in Ben 10'000 Returns, Grey Matter along with the rest original series and some new aliens that Ben never used before were unlocked by Ben 10,000, but 16 year old Ben never used Grey Matter in any of the episodes.
Original Series Aliens
Reboot Series Aliens