Well that's a relief thank you.What kind of fruit did you add? How much to how much mead?
Time is the mead maker's best friend.
I have an apricot melomel I put together December 6, 2019. It has been cleared, degassed, and stabilized but, it's still in bulk storage. Guess you could call it tertiary. When this went into secondary it smelled of moth balls (no smartass comments required). I'll let this sit at least six months before bottling then it will probably sit another six months. My point being, twenty days is nothing. Don't even begin to worry. Probably enough alcohol there to prevent anything growing anyway.
GOOD mead is not something you hurry.
Thanks for answeringA couple of questions.
You say you added fruit for nutrients at day 20. Did you add nutrients before this? DAP (Diamonium phosphate) or fermaid? Even some boiled bakers yeast.
Also is it a 1 litre batch? That's pretty tiny. What size and type of fermenter are you using? How was your sanitation?
Was the fruit fresh or dried? Did you pasturize them using campden or Potassium Metabisulphate?
Musty and sour can be a variety of things but usually either oxidation (exposing to air post fermentation) or bacterial infection. It could also be from stressed yeast. What strain did you use and how much?