#plus it doesn't help that since only Garmadon (and possibly the stone army) were able to see the overlord he couldn't talk to anyone

[toastingpencils37]

So it seems that Wu knew about the Overlord a lot more than Garmadon did, even though they were both raised by the FSM and knew Misako.

Which either means that Garmadon did actually know as much as Wu, but when finding the Overlord on the Dark Island, he didn't think of the information, whether because of the venom impairing his judgement, the Overlord manipulating him, or both.

Or like how Wu didn't tell the ninja about the Overlord in hopes that the name would never be said again, the FSM didn't tell Garmadon in hopes that it may prevent him from finding the Overlord and taking over the world.