ouch!
i'd take the sarco out (try to keep it submerged during the removal) and cut off the injured flesh in a separate container. dump the excised flesh and return the sarco back into the tank under some moderate flow, intermittent flow/wave if possible.
i tend to treat my tanks with Tech I or lugol's after such cuttings or fragging but that would be up to you. i would caution against lugol's if you're unfamilar with it. i'm using iodine in this fashion more as an antiseptic.
a waterchange wouldn't be bad either. the sarco's probably weeping up a storm in chemicals now plus when you return it to the tank, it'll be weeping a lot from the surgery. hth