Bum Crawler

Adjective; Someone who crouches behind a player and follows them around, usually using them as cover and/or to cause great annoyance.

Particularly prevalent on online FPS games, such as Call of Duty.

The verb is 'to bum crawl'.

Player #1: "Dude, look at that n00b behind you, he's been following you all game."

Player #2: "Yeah, he's such a bum crawler!"