At the high temperature at which this reaction takes place, a green-yellow solid may also be formed. propose a hypothesis about the chemical nature of this product.

i proposed that, since nitrogen makes up 78% of air, you could leave a crucible over an open flame for a long time to get it hot with an open lid to allow air in. thats because it is expected that you know magnesium reacts with the oxygen (burns) in the air to produce a whilte solid. 
your new experiment isn't doing anything different than attempting to delay the time before magnesium is allowed to react with the elements in the air (both oxygen and nitrogen)

it is the most commonly accepted one