If this datasheet is to be believed it is an Opto-Coupler. These components use internal LEDs and Photo-Transistors to act as "no contact" switches to control things without direct contact. This keeps more sensitive 0-5 volt digital logic from being damaged when improperly connected to AC or high voltage/current circuits, and so it makes sense to see one in a PSU. It would be my guess that it either failed as a result of corrosion/fault/shorting on the higher-voltage side, or simple breakdown of the component over time (seems unlikely). That goo may be unrelated.

It may be worth your time to take a look at stuff closer to the drive, are there any wires touching that don't look like they should be? And components a little too close to each other?

A more knowledgable member here may be able to offer wiser advice.