Moon doesn't have atmosphere, so once you've built the dish and cleaned it, it should stay clean unless a meteorite strikes nearby.
>"At sea level on Earth, we breathe in an atmosphere where each cubic centimeter contains 10,000,000,000,000,000,000 molecules; by comparison the lunar atmosphere has less than 1,000,000 molecules in the same volume."
>"We think that there are several sources for gases in the moon's atmosphere. These include high energy photons and solar wind particles knocking atoms from the lunar surface, chemical reactions between the solar wind and lunar surface material, evaporation of surface material, material released from the impacts of comets and meteoroids, and out-gassing from the moon's interior."
Source: https://www.nasa.gov/mission_pages/LADEE/news/lunar-atmosphe...
Also interesting, there is a thin film of electrostatically charged dust (regolith) that is visible across the moon's horizon during lunar sunrises and sunsets. The Apollo 17 crew drew a sketch to depict this phenomenon https://en.wikipedia.org/wiki/Atmosphere_of_the_Moon#/media/...
This is a reminder that he used to make sure we checked our assumptions and always considered constantly changing conditions.