magnesium alloy is not combustible.
Not true. Magnesium AND aluminum are both metal fuels used in the pyrotechnic industry. An alloy of the 2 also is a fuel. Either one makes some wicked compositions you DO NOT want to fool around with if you have no experience.
But, as an experiment, go buy one of those firestarters you scrape with a knife, file or scrape some off, and light it. Then, pour some water on it. Carefully, while standing back!