Allah Almostaan (الله المستعان ) means tha Allah is the helper. he is the one who we ask for help from. we say it spontaneously when someone asks you for a favor or help. or also if u at the end of conversation and your friend told you be around or we waana see you or asks you for anything , we say Allah Almostaan. it means we seek for help from Allah.
Actually allah almustaan not commen sentence but we use it when someone do something bad for us or say something not true about you من قال: أني سارق؟ الله المستعان يكذبون علي who said I'm thief? allah almustaan that is wrong they laying about me
لقد حرقوا سيارتي الله المستعان عليهم there are burning my car Allah almustaan
الله المستعان mean Allah (alone) Whose help can be sought