fleka

1. colloq. stain; spot
   Imam fleku od kafe na novoj košulji. — I've got a coffee stain on my new shirt.
   Ova fleka neće da izađe ni posle pranja. — This stain won't come out even after washing.
2. colloq. (figurative) blemish; stain on one's record
   Taj skandal je ostao fleka u njegovoj karijeri. — That scandal remained a stain on his career.

The neutral word is mrlja; fleka is more colloquial.