I recently made a ganache drip cake and I put fondant letters on top.  I waited until the ganache set but by the next day the letter had bled a little.  

I had put the cake in the fridge to set the ganache.  When I took it out I put the letters on a few minutes laters.  I’m thinking it condensated and that’s why the letters sweat. Or perhaps ganache drip is just too soft for fondant? The cake did not go back in the fridge.   

I’m doing a similar cake this week, wondering if you have any tips?  My piping (writing) is poor, so I do fondant writing.

Try putting a light color letters onto a darker color ganache instead of the other way around.  That should be much more stable.