I had that melting happen to me on an MP box about 25 years ago. It was an otherwise stock 383 Road Runner street car, with the MP ignition kit installed.
Never found a problem with anything but the thing oozing, replaced it with another (newer) ECU and no further problems.
Thankfully the mess cleaned off without issue.
I heard it was a bad batch of the potting material. I've used other parts-store ECUs on a couple other street cars, didn't have a problem.