I'm not too sure about it myself, but I believe a moonstone meteor spawned after I beat Brain of Cthulhu? There was no message to indicate it had appeared, I just happened to find it because it fell exactly into my Underground Desert entrance and clogged it up.

You might be able to find it by just roaming the surface after beating BoC or EoW. Might be similar to the original meteorite conditions where you need to pop a crimson heart/shadow orb.

I have just beaten BoC at least two in game days ago, and no moonstone has spawned

i think its just like meteorite