We learn from Aladdin that Mozenrath’s streets are decorated with these crystals on spires because they glow whenever something magical comes by. (Presumably magical things that Mozenrath himself didn’t make or his army would set them off all the freaking time.)
They’re here because Mozenrath wants to know when something magical that’s not him or his is wandering around his kingdom. I’ve expounded more than once on the notion that Mozenrath seems to take drastic steps to avoid a fair fight (or an unfair one that’s not in his favor), and whether it’s because he’s collecting or because he’s paranoid as Hell that other magicians are going to come and try to kick his ass, we’ll never know for sure.
The point is that he’s got one Hell of an alarm system set up on his property.
In this case, the crystals are glowing because Mozenrath’s new magic weapon has arrived in the city.
The DVD Fairy 