Designing Kitchens and Bathrooms - Can tile be applied over drywall mud?

I am finishing tiling in a bathroom . I have a 4 inch strip of unfinished drywall at the edge of the shower. The corner on the shower side has a metal corner, with the tile in the shower covering it, so I am unable to remove it. Because of the meal corner, this thin peice I am finishing is not flat and I would like to level it with the drywall mud....will I be able to apply the tile directly over the mud?? I am using mastic to apply the tiles, and the tiles themselves are 12" ceramic which I am cutting down to size. Would it be better to prime the mud prior to tiling over it?? If it is possible to tile over the mud, how long should the mud have to cure before proceding to the tile work?

Thanks!

Welcome to the forums. I would not use drywall compound for this. In fact, I would use thinset on my tile as well, but thinset is the best medium for the slight buildup you need. You are using concrete backer underlayment behind your tile, right?