First question.... chicks have a VENT, located JUST under the tail - high up on their rears. They also have a naval, located about 1/2 CM or so BELOW the vent. Is the stringy looking thing protruding from the VENT or the Naval?
It's not unusual to have the equivelent of the chick's umbilicial cord to not fall off completely and be haging from the naval NOT the vent. If this is the case, it looks like a really hard and dry string - just like a small piece of the excelsior bedding they use in the boxes to ship. If that is the case, then take a pair of scissors and clip the cord off close to the naval area. The rest will fall off on it's own, but trimming it will keep the other chicks from pecking and the chances it gets caught up on something and hurts the chick.
If it is the actual VENT that is blocked, that usually looks more like caked on poop and will be over the VENT (higher up) not the naval area. Take warm water and dampen a washcloth or papertowel. Gently scrub off anything blocking the vent area and then pat dry. You can also smear olive oil or vaseline around the vent to keep poop from sticking. Then keep an eye out to make sure it doesn't paste up again.
Let us know how she does!