Mysterious Slimy White Bubble-Like Growth in My Toilet Cistern – What Is This?

I recently opened my built-in toilet cistern and found this strange white, slimy, bubble-like substance growing inside. The tank has almost no ventilation or light, which might be contributing to this growth. It looks like some kind of mold or biofilm, but I’m not sure what it is. Has anyone seen something like this before? Thanks in advance!