American singer-songwriter who moved from country music to pop with albums like 1989 and is one of the best-selling musicians of all time. This is the nickname her Chinese fans use.
The 霉 here is not 'mould' but the 霉 of 倒霉 'unlucky': Chinese fans teased her for her run of bad luck, and doubling the syllable turned the joke into a pet name. It is now common enough that Chinese state media use it too.