Ahh, such a topic of... Mystery!
The use of them in our stories fills in the often missing gap, whether it be foe, mystery, or other.
Practically nothing is known about them, so we often end up creating them in our own ways, incorrect or not.

I've heard roughly 3 different ideas about the morai/spiral knights.
What colors do you think of at these names? Red and Blue, likely.
This makes me think that perhaps the knights and the morai were once part of the same group originally.

The three different theories I've heard basically boil down to:
They're robots!
They're humans!
They're aliens!

Personally, I think that the "robots" idea can be scrapped immediately. The mechas and the spiral knights look very different.
As much of a copout that one or both groups are humans is as an idea, it honestly makes things a lot easier.
However, there's also the likelihood that they're merely humanoid in shape, and are simply color swapped energy humans.
Energy humans. what a bad description. Glowey eyes? Of course, if they were humans, the argument could be that the glow of the eyes is merely a part of the armor's design. Speaking of the humanoid part, another valid response is that because the game is cartoonish in nature, anatomy can go right out the window, leaving it possible that they are in fact, human.

So, what I think?
I prefer thinking they're human, as most of the RPs and Fanfics I've been in have them as humans.
That's the standard or default, I suppose.

But if you want to go further into speculation, you could in fact have them as humanoid aliens.
Due to a lack of lore, my indecision should be understandable.