It’s little things like that image above that make comics so fun. Artists (and writers) insert all kinds of little references and in-jokes for perceptive readers to find. Some of them (like a Vulcan–from Star Trek– Green Lantern) make perfect sense.
The Green Lantern Corps is supposed to have a member from every sector in the galaxy, so a Vulcan GL Corp member makes sense. One little reference makes the comic universe seem so expansive and large and feel like anything can happen. For those that want to check his one-panel appearance, see Green Lantern 2nd Series (1976), Issue #90 – “Those Who Worship Evil’s Might.”