What creature is this?

I found this creature in a sample of water and duckweed collected at a pond of the Central park of NY. 

Apparently this creature lives under water, and it has a flexible body. 

It may shrink the body to about a 3-5 mm bulb.

And it can stretch the body to about 15-20 mm! 

It changed the color of the body seconds after I release it in the water! From later observations, I conclude that this creature has a semi transparent body.

Here is the closer up view of presumably the same or similar creature I noticed while recorded development of water snail eggs (that was the point of my set up - not the creature!). 

For comparison to the size of the creature: this plexiglass box has 4 mm walls.

Later on I moved 3 creatures in a smaller plexiglass box for further observation. The bottom of the box has a square shape with 30 mm sides.

The walls of the box are 2 mm thick.

I fixed the camera with manual focus under about 40-70 degree angle to the bottom of box. This way, the camera captures the view of the bottom and the front side.

There were some other tiny creatures in the same water sample! Never mind them - they are too small for my camera to take any decent record :(

Look at the mesmerising movements of these 3 creatures! 

The whole body has a shape of an arrow. 

There is a hole (mouth?) at the arrow's head. And you can see the inner parts (guts) in the middle of the body.

It has a flat body with thickness of a fraction of millimeter! Extremely flexible body moving effortlessly through the water!

What creature is this? Appreciate any help on this matter!
Have fun and happy bug hunting :)