It looks to be ballast or capacitor oil. If either are leaking, it will drip out the gear housing through the vent holes and the lamp holder. That should probably be seen to pretty sharpish.

BTW, this topic should be in lanterns or general discussion. This board is for the site

My guess would be capacitor oil as HID ballasts here are typically open core transformers which means they don't really leak. If that's a older highbay (pre 1980), you might want to avoid touching the stuff with your bare hands as it will contain PCBs.