One very important fact that we're currently lacking is the density of these moons. I have a gut feeling that these "moons" are not masses of rock like most natural planetary satellites in our solar system. I think they are actually enormous living organisms of some kind that are secreting these waterfalls of spores as part of their biological process.
If they are living creatures, then their density would probably not be anywhere near as great as it would be if they were composed of rock. Another fact we're not sure about is how far from the surface of Tress's World these moons are. The Lunagrees come across, in the text, like gigantic waterfalls of spores. For this effect to occur, the moons must be very close to the planet's surface so that the planet's gravity can pull the spores down in this manner. They also cannot be too far outside the planet's atmosphere, because I can't see how the spores could pour down they way they do if they have to travel any significant distance though the vacuum of outer space.