Is it okay to put a Moshi iGlaze cover on my Macbook 12 or will it overheat?

As long as the cover doesn't block ventilation it should be ok.

Learn about the fans in your Mac

"Mac computers have air vents that allow heated air to exit. The vents are in the back of the computer on the MacBook, MacBook Air, MacBook Pro, and Mac Pro computers. iMac (Mid 2012 and later) models vent along the bottom edge of the computer. Make sure the vents remain unblocked to allow your computer to perform at its best."

The new MacBook doesn't have air vents, and has no fans. It relies on its Aluminum casing to conduct away most of the heat. So, covering it would degrade that capability. Whether that would cause it to overheat is anybody's guess, but the MacBook's thermal protection circuits would ultimately reduce processing speed and eventually shut it down, if it overheated.

Fanless architecture

The new MacBook is the first Mac notebook ever without a fan. Since the Intel Core M chip draws only 5 watts of power and therefore generates less heat, no fan or heat pipe is required. Instead, the logic board is seated on top of an anisotropic graphite sheet, which helps disperse any heat that is generated out to the sides, all while your Mac stays virtually silent. The new MacBook is made even more efficient by OS X features like Timer Coalescing, App Nap, and Safari Power Saver, which ensure that every task is performed using just the right amount of power. No more, no less.