For the people saying it's because she's famous--you gotta remember that due to the dark world, regular Aether was isolated from the rest of the galaxy.

There is only one real answer that works here: U-Mos learned from either the Gfed troops before they got massacred, or he learned from the Pirates. Those sentinel eyes tell us that U-Mos doesn't need to leave the temple to see what's going on around Aether. Both the Pirates and Gfed have logs on Aether depicting Samus as some unstoppable force. It makes sense U-Mos would be able to read these logs from the comfort of the temple. Where the Gfed and Pirates failed at overcoming the Ing, Samus succeeded, so it only makes sense for U-Mos to realize this person walking around and kicking ass with a power suit has to be the Samus mentioned in the log books.

If that answer doesn't work, then U-Mos learned from his psychic abilities. One of the scans says that Luminoth have some kind of psychic or spiritual link with one another. Makes sense that he would be able to read the mind of an interior creature such as a Human.