Agreed, looks like poop, but it does look low. Can you see the vent above the yuck or just below the tail? If thr vent is visible and clear, I would leave it alone for now and watch closely. It isn't going to hurt to rinse it. I normally get a small, warm stream of water going and actually put the chickie butt under the faucet. Sometimes dried poo needs to be broken up a bit to remove it, but you do need to be careful not to actually pick the umbilical scab off. I hold the chick in one hand and gently rub with the index finger of the other hand.
Umbilical cord. Vent higher than that. Don’t remove it let it fall off. Keep an eye on it, a larger umbilical scab will assist helping poop to stack up and clog it. Should be long and it will come off. Vent above it so picture looks. Go all the way to very end and you should find.