Obviously they have no clue what the person with MS is going through..."looking sick" is an observer's way of understanding the person is really ill.
I would say thank you.
and then, probably follow up with a mild sentence or two about the disease, depending on how close they are to you. They don't mean to be unkind I'm sure, it's just that they are ignorant of your challenges.
My mother heard "You don't look blind" all the time...and she just decided it was a compliment and simply replied..."Thank you." sometimes, depending on how sassy she was feeling, she would retort "You don't either."
just forgive them and look for the funny, it's salvation for despair.