That’s fine and normal. The owner was too lazy (for 30 years) to wipe it off.
The steering knuckle cavity contains grease that’s sort of retained by seals. Even new or rebuilt steering knuckles weep a bit of grease as they should. This vehicle is also weeping a bit of gear oil too that has migrated past the inner axle oil seal (normal).
If this is the only “red flag” you’ve seen on this vehicle, it’s not a red flag.
If you purchase it, just clean it up with a rag & gloves and add more grease to the knuckle. Betcha it’s dry.
Also check the oil level in the front differential housing. (If you buy it).
Eventually you’ll want to reseal the knuckles (if you buy it) but it’s not a immediate concern.