What are the "icy slush" hydrates that clogged the box they put over the leaking well in the Gulf of Mexico?

methane hydrates or methane clathrates.

Wikipedia has a nice article about them: http://en.wikipedia.org/wiki/Methane_clathrate

The 'Icy slush' consists of complicated molecules of Hydrocarbons and Water of solubility as hydrates.

They are semi-solid masses that form when hydrocarbon gases containing water vapour are decreased in pressure resulting in refrigeration (Joules-Thompson Effect).

They are generally an undesirable product which can clog up piping and fittings in a process system. They form at temperatures below 18°C.

I have worked on a Gas Plant that purposely produced Hydrates in Natural Gas by decompression of high pressure gas in order to remove water by Low Temperature Separation operations.

The hydrates are separated from the gas and gas condensate by a process using hot water coils in a special separator which also contained various baffle systems. The hot water coils melted the hydrates which floated on liquid hydrocarbons into more liquid hydrocarbons together with water which were then separated by the baffle systems.