Man, this was the first post I made... Anyway, I think I've come full circle and back to the same conclusion.
The second moon is Dreamstone. A massive quantity of Dreamstone. I'll lay out the evidence why.
In the Keystone timelines (i.e. before the Time Crash), we see Dreamstone in two eras. First in Prehistory, where it is "rare" but still plentiful enough to be revered by the natives and used in weaponry. Secondly in Zeal, where it is used to craft various magical artifacts, each with corresponding Spirits; the Masamune with Masa and Mune, Schala's pendant with Doreen, and the Mammon Machine (with possibly itself as its own Spirit). There's also the Frozen Flame, which has decidedly Dreamstone-like qualities; it is most likely contained within the Mammon Machine as well since, in Radical Dreamers, " the precious stone was in [Schala's] possession" at the time of the Ocean Palace Incident, and her pendant is clearly not the Frozen Flame. Beyond that, Dreamstone is not spoken of, and Robo states he has no records of it at all from 2400 AG.
So with this, we can tentatively conclude that Dreamstone is disappearing as time goes on. Why? I suspect Lavos is the culprit since she enters the scene in Prehistory, where Dreamstone is most plentiful. And, I suspect, the name given to Lavos in the secret ending is more accurate to what Lavos as a species actually is: A Dream Devourer.
Now combining this with the original idea I shared 11 years ago--that the Dragonians created the moon. With no Lavos (i.e. Dream Devourer) existing in the Lost Sanctum timeline, it follows that Dreamstone doesn't disappear. And given what we know of the Dragonians having lived more in harmony with nature, I argue (though it is far-fetched and simply theory at this point on) that the Dragonians were able to cultivate and harvest Dreamstone throughout the eras with "a biological machine used to control the powers of nature". Eventually, this machine would, with such vast quantities of Dreamstone, become a "plasma life-form" and "a living accumulation of the planet's energy"; in other words, the Dragon God is also a Dreamstone Spirit just like Masa, Mune, and Doreen, with the second moon being its vessel.
It might also explain the vague description from Belthasar that the Dragon God "was consumed long ago, in the distant past... Integrated by the entity known as Lavos in a time on the other side of the dimensional darkness". Note he says Lavos and not Time Devourer (the latter part, "in a time on the other side of dimensional darkness", I think could just as easily be interpreted as Lavos' pocket dimension instead of the Darkness Beyond Time). So Lavos could be "consuming" the Dragon God by way of absorbing the second moon's Dreamstone (the "actual" form) while we still face off against the Dragon God's Spirit form in Chrono Cross.
And all of this neatly ties into an overarching theme of the Chrono series... Dreams.
Also the moon is fricking blood red like all Dreamstone.
EDIT: Also, if Dreamstone is the source of Spirits like Masa, Mune, Doreen, and possibly even the Dragon God, it conveniently explains the Entity's existence too: That is, the Spirit of the planet. Lavos feasting on the planet's Dreamstone is actively killing the Entity, hence the reason for the Gates so that we can experience the Entity's life.