In the latest chapter we saw Gray switching to demon slaying mode which is shown by the appearance of the black markings but he didn't use his demon slaying magic instead he used his regular ice make magic. Why?
They're separate magicThat ice magic had demon magic embeded in it.
gray used his demon magic in tartoras arc where it looked exactly same like ice magic too. So the diff. is it is stonger form of ice in demon form but when you are in normal form it became the normal ice making magic.They're separate magic