Cessna 172 – Strange Green Goo Under Wing

Strange green goo found under right wing – seems to be related to fuel system. We sumped the tank twice and it was blue 100LL with no contaminants both times. We wiped it clean and flew an hour in the pattern. After that flight there was no sign of it. So it remains a mystery as to where exactly it came from. We've had control over this plane for 2.5 years now and we only use 100LL.