The one and only mosque that we visited in Siem Reap was the Naekmah Islami Mosque. The building is located in front/beside the Muslim restaurant we went for lunch. We had the chance to perform our afternoon prayers there. I believe there is a Muslim community in the area.
Behind the mosque, there is a small "madrasah" [school]. I did not visit the school as I did not have enough cash with me to give donation. I was just afraid people were expecting visitors to donate and I did not want to let them down.
It always makes me feel like home to see a mosque when I travel - something which is just indescribable.