They need to return to a hospitable aqueous environment to reproduce, i.e. the earth.
What does this mean? It can never be a sustainable colony, and their biological material is never regenerated.
Exposure to low-level radiation (eg fallout) integrates ... For low level radiation you are not killed by how many rads/second you receive (radiation intensity), you are killed by total rads integrated over time (dose). Because unrepaired mutations accumulate over time.
So if not reproducing, your biological material can only go in one direction: Downhill.
So if, instead of spending 24 hrs on the moon you are stuck on the moon for a year, or 10 years, or 100 years, or 1000 years, or 10,000 years, or a million years or a billion years and can only reproduced after returning to earth, then by definition you are no longer on the moon, and what remains on the moon is progressively degrading.
... It doesn't really matter how hardy your tardy is, it could be 1000x more hardy than a human, but after a thousand years, the genome will have been picking up more and more mutations. These will sterilize it. It will turn from biology into chemistry only.