Green sticky goo.

Fuel leak that is dissolving the glue of the anti chafe tape covering riot heads under your fuel bladder.

Greg is correct, the residue is from the dye in the fuel after it mixes with the tape in the tank bay. If it only leaks when it is full, the issue is probably a vent nipple on the outboard end of the tank cracked or broken. Only repair is to remove and replace the tank.
If it is leaking when partially full, try checking your fuel sending units and see if they are tight. Sometimes the gaskets will shrink slightly allowing fuel to seep out and run down the tank bay to the lower end.
Depending on what model you have, you may have one or two sending units per tank.

Bob Ripley
ABS Senior Technical Advisor