If you have green coraline, conditions are right. The purple (and orange/red and yellow) will come. Just be patient. Don't dose Mg (or anything) without testing for it first. If this is a new tank, I would be surprised if you had to dose anything at all. Regular water changes with a good salt mix should be all you need (especially if you don't have corals or just have small frags). Algae like chaeto should be trimmed to complete the nutrient export process. If the algae become too matted it can potntially do more harm that could (i.e., detritus trap).