Garlic turning green?
As far as I'm aware there are a few possible reasons for this to occur.


Young garlic can turn green when the presence of an acid, in this case the butter.
As a possible chemical reaction between the garlic and certain types of cooking utensil, such as cast iron or copper.
It will sometimes change colour if it has prolonged exposure to bright sunlight.


The most important worry can be dismissed, it's not harmful :)