What causes the Air conditioner to blow black greasy foam out of the vents? on 2002 Cadillac Seville

The A/C works fine except that it now blows black greasy foam out of the vents everytime it is in use. What causes this and is it worth fixing?

Asked by for the 2002 Cadillac Seville
Only thing i can think of is system oil leaking from evap. core, if it is greasy, but should also be leaking out onto the ground as well. It's worth fixing if you need AC and in SC. i bet you do! Have the ac drain checked!
Well I look and their is nothonh leaking on the ground.