ID on this hitchhiker?

[liz_au]

This guy has been growing for about 8 months now, started off so small I didn't even notice him. Any ideas on what it is?

- Liz

http://i.imgur.com/Hh95Aqn.jpg?1?6128

[liz_au]

Holy balls large image.
Try this link it might work better.

http://imgur.com/Hh95Aqn