Because of the Omnitrix being recaliberated in Innervasion, Slapback has replaced Grey Matter in Ben's set of 10.
When struck on the back with enough force, Slapback splits into smaller yet denser copies of himself. In this alien form, Ben can become his own mini strike squad! The more of him there are, the tinier they get, but don’t let size fool you – they’re as strong and solid as rock!
301 - "Omni-Copped" (first appearance)