Is Magnesium hydroxide a strong alkali

No, Magnesium hydroxide is a medium strong alkali.
The acidity and alkalinity of a chemical substance are relatively determined by the degree of ionization in its aqueous solution. The more active the metal, the greater the degree of ionization in water.
The stronger its acidity and alkalinity, the Magnesium hydroxide can only be a medium strong alkali (Magnesium hydroxide has a very small solubility, the solution is very weak in alkalinity, and sometimes it is treated as a weak alkali) because of its Degree of Dissociation in water. Heating to 623K (350 ℃) means dehydration and decomposition: Mg (OH) 2 → MgO+H2O, which is easily soluble in acid or ammonium salt solutions.