Hexy is a slang abbreviation for Hexamine, a kind of solid fuel which allegedly (according to the powers that be) burns cleanly and leaves no residue or ash.
Hexamine, given enough encouragement, will burn, releasing copious amounts of noxious fumes, its true there is no ash or residue left on the ground, this is because the residue has migrated to your mess tin, which is now coated in thick immovable black goo.
'Hexy Telly' is the recreational use of hexamine, and many people claim to be able to read the future while in a hexy telly (fume) induced trance (quite often the future seems to consist of a mess tin coated in thick black goo and a lukewarm cup of tea, but you can't have everything).
Useful for replacing donations of sweets to the natives. Hexy and jam sandwiches are most nutritious.
Also popular with street fighting urban guerillas and the Luton branch of Al Qaeda since it can, with the addition of very few, fairly common, household chemicals, be used to manufacture some very powerful explosive compounds.
Believe it or not, another form of Hexamine is used for the prevention of urinary tract infections, where it reacts with urine to form Formaldehyde, which apparently, is an effective antiseptic against bacteria and fungi. So next time you've got a few spare blocks left after exercise, jam them up your japs eye before going munter hunting as a precaution against catching anything nasty.