"Yes, mother already is in moscow" is what I just answered and it marked it as incorrect. Я Не знаю if it should be correct or is actually wrong.
...and two years later, it's still not accepted. Reported it.
(I entered "Yes, mom already is in Moscow." - slight difference, but anyway...)
Москве is Locative (prepositional) or Dative case of the name Москва. In this case it's locative.