I'd change what thegrun said just a tiny bit.
Too high isn't an issue (up to a point of about 2200ppm anyway), too low can inhibit stony corals (both lps and sps) from taking Ca and alk out of the water to make skeleton. Higher levels of Mg along with alkalinity and calcium at higher levels MAY ALLOW your stony corals grow faster. There is no guaranty that they WILL grow faster as there are lots of other factors involved as well (other water parameters, flow, light, food, etc)....:beer: