Strange Blue Ball

[A. Iosue]

I have found a blue-green ball about 1/2 inch diameter inside the aquarium. It is so light that low water flow can move it. If you see the pictures in my gallery: Blue Ball and Blue ball crop, you will see that it traps organic material from substrat, also it have something transparent-blue like a skin wrapping around it, perhaps it could be seen in the crop.
A crab have puntured it and start to emanate a light blue liquid as dense as a caramel in water. Just in case, it is not a candy!!!!
I would like to know what is this thing. Any help will be appreciated.