From the Super Mario Wiki
Shroob Chomps are enemies that appear in Mario & Luigi: Partners in Time. They are only used in an attack made by Elder Princess Shroob. It looks almost exactly like a Chain Chomp, but it is purple and has Shroob eyes. When used, Elder Princess Shroob lets go of the Shroob Chomp and sends it at Mario, Luigi, Baby Mario and Baby Luigi. The Bros. have to jump repeatedly until a Bro. stomps on it. It then drops out of the screen and out of sight.