Get on the ground and inspect that pulley much more closely.
Start with the engine running, and use a flashlight to look at the rotation of the pulley. Is it wobbling? If so, see BELOW.
Then, with the engine off, inspect the harmonic balancer retention bolt which is attached to the end of the crankshaft. Make sure it's still there, and that it's tight. If it's not tight, see BELOW.
Then check the bolts for the pulley attached to the balancer to make sure they are tight.
If the harmonic balancer is pulling away from the engine, the front seal could begin to leak as the front seal, seals against the hub of the harmonic balancer.
If everything looks o.k., I wouldn't be surprised if the oil is just running down from a valve cover.
BELOW
Our cars have what many people would consider a defective design in the harmonic balancer. It is an interference fit on the crankshaft, but is made out of sufficiently soft material that as the rubber in the balancer deteriorates engine vibrations can distort the hub. This distortion allows the unit to start to walk off the end of the crankshaft, typically breaking the retaining bolt leaving part of the bolt in the end of the crankshaft. Options are to replace with a stock unit, or the single aftermarket alternative. The stock unit is the same questionable design of what you have right now.
You can search the forum for "harmonic balancer" and find many a saga of failed units.