Gelatinous cubes only eat people (and treasure and gear) because they're lonely, and like many emotional eaters they have a hard time finding a love who will understand their situation and won't judge them.
But green slime don't judge, and since both slimes are sexless and ambitionless masses who feel existence has no meaning you'd think the two dungeon dwelling creatures would make perfect partners.
However, as this comic from Joshua Wright shows, when lovers discover they have everything in common, including their slimy complexion, it can be hard to tell who's who!
-Via Geeks Are Sexy