Flim Flam was a con artist who convinced the gang to go on a trip to the moon, using a fake alien invasion as a ploy to steal a valuable moon rock. The gang eventually unmasked him, but he managed to escape with the rock.
Flim Flam's fate was never explicitly revealed. He didn't appear in any further episodes. We can speculate that he continued his con artist ways, perhaps using the stolen moon rock to pull off even grander schemes.
It's worth noting that Flim Flam is not the only con artist to appear in Scooby-Doo. There have been several other characters who tried to fool the gang with their schemes.
If you have any other questions about Scooby-Doo characters or episodes, feel free to ask!