- In a purely scientific sense possibly not (though the difference between the two might be considered more "breeds" than "species," hard to say on the evidence presented). However from a moral standpoint eating another sapient species can be considered cannibalism. The Eloi, while stupid (or possibly just lacking in curiousity) are capable of speech and wear clothing and otherwise have a society, albeit a rudimentary one. So the Morlocks are still killing and eating people even if they aren't of the same species.
- The eating of people is 'anthrophagism'. Cannibalism is specifically the eating of one's own species. If the Morloks and Eloi are as genetically different as, say, chimpanzees and bonobos ('Pan troglodytes' and 'Pan paniscus', respectively), then Morloks eating Eloi is not cannibalism regardless of whether or not it is immoral.
The talking rings mentioned how some of the survivors stayed above ground. Having lived through a centuries-long nuclear war that left the entire world barren and trashed, these survivors likely developed serious contempt for their ancestors who destroyed a world out of ideology. To prevent another such war from happening again, the survivors built a society free of ideology, borders, or history, seeing those things lead to wars that were destructive enough to bring humanity to the brink of extinction. Along the way, abandoned things like industrialization, science, and learning, since so-called "progress" helped create more death and violence. They may have even given up farming in favor of orchards that would serve as a common food source for their settlements.
The ancestors of the Eloi created a post-primitive society where everyone would do nothing but play, skip, and enjoy the fruit.
And this worked for a time: war stopped, the biosphere eventually recovered, and society became peaceful and relaxed.
However...
So when the world's biosphere recovered, the Morlocks eventually returned above ground and found not only a pristine planet but the Eloi, pacifist vegetarians who didn't believe in violence.
In other words, the perfect food source to their cannibalistic brains. Being the product of a Darwinian struggle, the Morlocks had both the strength and the will to cull the Eloi for a meal.
The Eloi, after living in their supposedly utopian and peaceful society, would lack any real self-preservation instinct nor the ability to recognize a potential threat to survival. Like the dodos of Mauritius, they weren't exactly dumb, but living in a cozy environment left them without any understanding of what was and what wasn't a threat. Eating only a fruitarian diet devoid of protein likely weakened their physical and cognitive abilities.
Over the millennia, the Morlocks gradually conditioned the Eloi into being their cattle, even using the air sirens as a way to lure them underground in a Pavlovian-style act of conditioning.