What Is This Creature On My Plant?

I just got some new plants in the mail today and after getting them all settled in a noticed what looked like a dark green bud on one plant. Then I noticed it was moving on its own. It has a little "arm" that it sticks out from either end to grab onto the plant and move itself like a clam, but it doesn't look like any type of clam shell that I've seen or tried to find on the internet. When it released from the plant it floated to the top so I removed it and put it in a separate container. The little "arm" almost looks like a worm, or maggot. Maybe it's something that doesn't even belong in the water. Can anybody tell me what this is?
 

 

 

 

not a clue but glad you've removed it ... looks horrible 
 
 
 to the forum though

Hi, how long is it? do you have any videos that you can post of it?
its quite hard to even see what Im looking at..... nothing that Ive ever come across!

It looks like some sort of larva emerging from a pupa. Look at the right. I see what looks like an insect head, body, and legs coming out.

Looks like the larva of a caddisfly. They are harmeless for your fishes, they will damage your plants, though. So it's probably the best you removed it from your tank.

http/www.lifeinfreshwater.org.uk/Species%20Pages/Caddis%20flies.jpg.html
 
this looks like your pic. Caddis Fly larvae pick up debris to make a protective covering.