Peace be upon you. If the clay sticks to the forehead after rising from prostration, it is necessary to remove the clay if the validity of the prostration depends on it being separated from the forehead. According to Sayyid Sistani, if one prostrates on the clay and it sticks to the forehead during the first prostration, it must be removed for the second prostration, and it is problematic if one does not separate it from the forehead and prostrates in that condition.