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.