The script is silent on it other than C3-PO being "disheveled".
Jabba immediately flies into a rage, knocking the golden droid off the
raised throne into a clattering heap on the floor. Boushh adjusts his
weapon as Jabba raves in Huttese and Threepio struggles back onto the
throne. The disheveled droid tries to compose himself.
The Official Novelisation doesn't even refer to Jabba's arm but rather his tail.
Threepio quietly translated for Jabba, who immediately became enraged and knocked the golden droid off the raised throne with a sweep of his massive tail. Threepio clattered in a heap on the floor, where he rested momentarily, uncertain of the correct protocol in this situation.
So, we don't know but apparently it's not from Jabba's arm.
Here's the image of C3-PO just after he's hit. There's nothing on his chest or face. Note that he wasn't hit on the face but when he get back up he does have green goo on it.
It seems likely that, whatever it was, he picked it up when he landed on the floor.
Here he can be seen with goo in three places