It's technically called the front crank seal.....Lawsoncl said the harmonic balancer needs to be replaced....thats good, not nearly strong enough wording.....It MUST be replaced IMMEDIATELY!!!!....the outer ring can fly off and destroy more stuff under the hood than you can imagine....additionally, it cannot do it's job which as the name implies is to dampen harmonics...in other words....if it is bad, your crankshaft and main bearings are not happy...
You will need a puller to get it off....remove the belts and stuff, remove the bolt, take off the big washer and screw the bolt back into the crank so you dont mess up the threads in the crank, install the puller and pull the old damper off...pop ou the seal, lube the lip of the new seal with grease and carefully drive it in place....install the square key in the slot, make sure everything lines up and wiggle the new damper on as far as possible....now you need a pusher tool to push the damper in place....do not hammer on it...take the bolt that goes into the crank, go to an auto parts store...if you have an autozone nearby and they have the tool, you can rent it cheap...just be sure you have the right size and pitch...do not force the wrong tool into the threads....push the damper in place, install the original bolt and big washer, torque it down...reinstall the belts, etc....note this can be done from underneath the rig...hope this helps.