Hello!
The reaction you are seing is the solvent migrated from the putty to the fuselage parts. I'd say tossing that putty would be a serious mistake, because the reaction like this talls you the putty has a solvent in it that's perfectly compatible with the kit's plastic, and that means very good adhesion and even shrinkage - those are good signs. What was probably your mistake was that you used too much of the stuff in one place - that's never a god thing to do with any putty. For applications like that you're much better off using some kind of two-component compound, some epoxy glue or milliput. With two component compounds you have to watch for heat generation, though.
Now your fuselage will be all right after the solvent evaporates - and that will take some time, maybe even three or four days, but the plastic will be hard again. Try to keep it in shape until then, and when it gets hard again you can putty it and sand smooth. OK, it's a setback, but I believe you can still save this one. Good luck and have a nice day
Paweł