• half a mana can't be spent on full mana costs
• two half manas of the same color automatically become one mana of that color, which can be spent normally
• half mana will not expire when steps and phases end. When they become full mana, they will expire normally.

But the important question is: Can I spend two different half mana together to pay for a cost of the appropriate hybrid mana?

The fact that half mana won't expire, but the resulting full mana will is a trip. If you are going that path I'd rather go a different route entirely e. g. ": Charge . (If you have a green charge, use it up to add . Otherwise, gain a green charge. Charges don't expire.)"

Now that would be a conceptual divide to help grokk the difference.

I'm also a little confused from your comment whether you intent half mana to inherently not expire, and if so, why it's rules text on the card rather than reminder text.

Another alternate wording:

Mox Serpentine
­
Artifact - Mox
Tap two untapped Mox: Add .

By the way, I really like this take on the moxes. It has the power level down just right. I'd also suggest other ways in set to produce half-mana, so you don't feel so bad when you only play one Mox (This is pretty much a dead card in draft, for example.)

Funny thing about my suggestion to add more half-mana to your set. If you do, my suggested "Tap two untapped Mox" becomes a terrible idea. I mean, technically you could add something like...

Mox Dog
­
Artifact Creature - Mox Hound
2/1

But I wouldn't suggest this being the way out. [Of note: Mox Dog works because Mox is an artifact type, not a creature type. Yadda, yadda, "The Tribal Card Type" yadda yadda doo. You get the idea.]