Wa alaikum al-salam wa rahmarullahi wa barakatuh
Meat that is not slaughtered according to the standards set by the Shari'a is classified as carrion, which is considered impure (najis). Anything that this impure meat comes into contact with (with the existence of moisture on at least one side), becomes contaminated (mutanajjis). This includes utensils, the grill, and other meat. Anything (including meat) that becomes contaminated must be purified with water.
If the halal meat is to be cooked on the same area of the grill where carrion was cooked, then the grill must be cleaned prior to placing the halal meat on it. In the event that the halal meat was placed on the grill without cleaning it, then the meat must be washed before it can be eaten.
Allah knows best
Answered by Shaykh Omar Mohsin