These are Champa Malays, who came to Cambodia several centuries back from Annam, now in present-day Vietnam. Why do they speak Malay? How did they end up here? Is Pengkalan Chepa in Kelantan a Champa Malay settlement? Do Kelantanese originate from Champa Malays? Read THIS, THIS and THIS.
The ones I see look exactly like any Malay in Malaysia ...
... except they converse in Khmer.
Only a few elders can still speak Malay (with a distinct twang, and lack of vocabulary), and also Cham, their other mother-tongue.