Here are my ideas if who it could be
1. The G-Man got tired of stalking Gordon Freeman, so now he is in disguise and is stalking you
2. It's may be like the Ghosts of Halo 3 (Lag + connection glitch), only in Blockland (Ghosts of BL :D), does it follow you?
3. Glitched corpse
4. You, and you are just a liar
5. Your shadow?
6. Steve Urcle
Eh. i have no idea, that gun i know, it shoots purple spiral things, i forget the name, i use to have it on my server though