MgCl is not Magnesium Dichloride because of the word “Dichloride.” The prefix “Di” represents “2.” And “Tri” represents “3,” and so on. Since Cl (chloride) is only 1 in this formula, the name is “Magnesium Chloride.”
If the formula was MgCl2, then yes, it would be Magnesium Dichloride.