Not being able to access your sub account even though you have been recognized on the forum like in geometry dash lite, sometimes you ne ed to find a new path so try using the 'Forgot Password' option for your game account.

It seems that game accounts and forums handle logins differently—you should try contacting the game's official support to have them check, Gamma Emerald , and restore your access! 

Sounds frustrating! If the platform still recognizes your alt, try clearing cache or reinstalling the app. Also, contact support with proof—like your FB/Google info tied to the alt. Good luck!