Flag is provided by an API endpoint.
API endpoint location is in the Java source
API key is in res/values
MD5 the key, pass that to the endpoint and collect flag + points.

https://advancedpersistentjest.com/2018/02/24/writeup-flag-checker-2-mario-mystery-envy-xiomara-ctf/

Original writeup (https://advancedpersistentjest.com/2018/02/24/writeup-flag-checker-2-mario-mystery-envy-xiomara-ctf/).