I never heard of "light" being used in that context, but Marine Infantry generally refers to Seebataillon troops. Along with Matrosen regiments they formed the Marinekorps Flandern, which as you can guess by the name was stationed for pretty much the entire war at the very top end of the line, on the Flanders coast opposite the Belgians and Brits.
While at the start of the war they fought in their striking dark blue uniforms with light-infantry style shakos, they were in field grey by 1915 and by the end of the war only really distinguishable from a German army soldier at a glance by the gold litzen on their collar and their white shoulder straps with gold anchors.
The Matrosen fought in their blue sailor uniforms to start with, but you see them in field grey later, too (both field grey sailor suits and army-style uniforms).
Again, don't know where the "light" comes from. Might just be because the Marine Infantry uniforms made them look like Jaegers. Maybe I'm barking up the wrong tree here, entirely, though.