What the !@*( is this in my fish tank?

Okay, I admit, I have not done a water change for about a month. I’ve been busy! However, all that is in there right now are tons of plants which have been established for over a year, and one Minnow. It is a 200L tank.

Anyway, I was sitting next to my tank when I saw this THING crawl down the inside glass. I have NOT put anything in my tank for months. I am now in a panic that there are some weird mutant toxic creatures growing in my fish tank. This would not be weird, except for the fact that I’ve put NOTHING new in my tank for at least 6 months. Please take a look at the picture and help me diagnose what the hell this weird creature is. I would suspect fish babies but it has LEGS! ACK.

http://i55.tinypic.com/29dke46.jpg

Any ideas??