You break it lik any other block and receive nothing in return. Hooray!

 

It is a monster spawner inside of a dungeon. And by the looks of it, that will spawn skeletons if the light level 7 or below in that area. If you ever see mossy cobble or regulr cobble it is probably a dungeon that holds chests and a monster spawner. Skeleton, spider, cave spider, and zombies spawners all spawn naturally, but cave spider spawners are only found in abdanoned mineshafts, and on ECC they dont poison you.