Challenges appear after you clear every song on any difficulty. Challenges are random and appear once you clear multiple songs. Songs are determined by ones you have already cleared. The challenge score is determined by rounding your latest score for that song up to the nearest 10,000. You must pass this score in order to successfully complete the challenge.
When a Challenge Appears
Challenges randomly appear after clearing songs in the game on any difficulty. These challenges only appear on songs that you have already played and scored on. The challenge score (the score you must successfully obtain in order to succeed) is your current score rounded up to the next 10,000. For example, if you scored 1,278,430 on any given song, your challenge score would round up to 1,280,000.
You are given 3 tries to beat the challenge.
If you successfully beat the challenge, the rewards are RP, a random C card, and a Headphone.
Failing a Challenge
If you cannot beat the score, it tells you "Mission Failed" in red letters and 'booing' is heard as the background music, but you will still receive your normal song reward of 400-550 RP.
Failing a challenge 3 times will not grant you challenge rewards, and will only grant normal song rewards. Failing a challenge does not lower your scores or affect your stats, cards, or inventory in any way.
Clearing a Challenge
If you are able to beat your high score, it will say 'Cleared' in a light blue color and cheering is heard as the background music.
Note: The stars you achieve have no influence as to whether or not you complete the challenge, only the score is judged.
The RP received ranges from about 3,000 RP to around 8,000 RP. The total amount seems to be somewhat based on your cards and score, but overall, the number of RP is roughly randomized.
In SSM, there are chances for rare, seemingly random songs to appear. Currently, there is one active hidden song, the full version of Into the New World by Girl's Generation. The cards for hidden artists (artists that are not in normal gameplay) are unobtainable in gameplay, and in hidden stages where the artists are available in normal gameplay, the player's decks for artists are unusable because the hidden stage cards are classified as ______(Hidden), similar to ____ (Event) cards; this renders the player cardless in the hidden stages which exceptionally lowers the score of the song. These songs are only available in Hard mode with no regards to the player's set difficulty (Easy, Normal, Hard). These songs are considered some of the hardest in SSM due to their length and natural difficulty, with the retired hidden stage "Eleven" once officially being named the hardest song in SSM.
It appears there are specific requirements to "find" these stages, such as completing/failing pre-existing challenges on certain songs, having certain cards or card levels, successfully receiving 3 stars on a challenge song, or maybe even a specific time period during the day when the songs are active; however, these requirements have not been confirmed and are seen only as speculation.
The reward for completing these stages is RP and a C card, which are the same rewards for non-hidden stages. Hidden songs are not viewable in the song selection menu, even if the song is successfully completed; however, if the song is within your Top 5 for a week, it will count towards your Weekly League score and appear within the Top 5 in your profile menu.
Retired Hidden Songs
Eleven by Hitchhiker, Around by Hitchhiker and Taeyong, Hope from the SCHOOL OZ OST, Tok Tok Tok by Super Junior-T, the trot subgroup of Super Junior, and the full song of Rookie by Red Velvet are all former hidden stages that have been retired. Tok Tok Tok reportedly allowed players to use their deck for Super Junior to complete the song (though no videos or pictures have been found to support these claims), but Rookie did not allow players to use their Red Velvet deck because it was classified as Red Velvet (Hidden), similarly to Event cards which are classified as _____(Event), and there were no cards available for SCHOOL OZ or Hitchhiker's songs. These songs, like their active counterparts, were only available in Hard mode, regardless of the player's set difficulty (Easy, Normal, Hard).