Mortar over Mastic

I have small tile project going here and thought there was enough mastic in the bucket but there isn't. I stopped about half way and cleaned it off. Backer is cement board.

Now the question is I have some mortar here and plenty of that but will there be any issues with the mastic residue? Guess I can try and sand it off a little just to make sure.

I would go and buy some more mastic. Not that expensive. Mortar isn't waterproof and usually needs a thick bed to work. Not an expert but I know the pain of using the wrong product.