Browsing through some pics of Green Lanterns through the years, there have been some that wore a green leotard over the black bodysuit, so perhaps that could be one way to go.
Or, if you really want the cut-away look, perhaps pick up a green, skin tight t-shirt (or perhaps something like UnderArmor for a more sleek look), cut it to match the green layer in the costume, and seal the edges of the t-shirt to the black bodysuit using something like Heat N Bond or other fusible interfacing (basically, an iron-on glue between the layers).
Unless you're going to be using foam "musculature" padding (I have zero idea what your physique is like), which could go between the green shirt layer and black bodysuit.
I'm certain others will come up with better ideas, but that's a start.
Or, if you really want the cut-away look, perhaps pick up a green, skin tight t-shirt (or perhaps something like UnderArmor for a more sleek look), cut it to match the green layer in the costume, and seal the edges of the t-shirt to the black bodysuit using something like Heat N Bond or other fusible interfacing (basically, an iron-on glue between the layers).
Unless you're going to be using foam "musculature" padding (I have zero idea what your physique is like), which could go between the green shirt layer and black bodysuit.
I'm certain others will come up with better ideas, but that's a start.