Grey Matter is a Galvan from the planet Galvan Prime. He is Ben's 5th transformation who appeared first in Washington B.C.
Grey Matter is a very small alien. Its unknown how small he is but he is the smallest alien in his galaxy. He is all greenish and looks like a martian. He is wearing the same shirt that normal Ben is wearing and the Omnitrix symbol is on his back.
Being very small grey matter is able to go through the smallest of spaces giving him an advantage. He is also the smartest being in 5 galaxies so he can create almost anything or destroy anything.