Croc can't leap in pursuit, because the Batman's latched onto his tail like the largest parasite in the world. He lashes out at Frisk for a few more seconds-- roaring all the way, until he slips back on the slippery mud.

Then he turns on the Batman once Frisk is clearly out of range, free to flee and Croc foiled. (For now.)

"You made a big mistake, bat."