Mobs cannot spawn on blocks that are less than a full block in height. ... Mobs will also not be able to spawn on top of transparent blocks, such as glass, or partially transparent blocks, such as leaves.

What can mobs not spawn on?

Most mobs never spawn on transparent blocks, in water (except fish, dolphins, turtles, and other aquatic creatures), in lava (except for striders), on bedrock, or on blocks less than a full block tall (such as slabs placed on the bottom half).

Does glass stop mobs from spawning?

Hostile Mobs cannot spawn on Glass. Mobs can not look through Glass since Glass is a solid Block.