Although dyscalculia is an accepted condition/label and has been defined as " Students with dyscalculia have difficulty understanding simple number concepts, lack an intuitive grasp of numbers, and have problems learning number facts and procedures" I have never been clear how it differs from 'not being good at arithmetic'.
It's not uncommon to be good at some things but not others. I can't read music or draw but I'm not sure a label (if there is one) would help