I've been reading up on Calurepa and other macro algaes. Just like freshwater plants, iron is beneficial to its growth/health. Is it bad to add the plant fertilizer I use in my freshwater tanks to my salt tanks to help the algae out?
I wouldnt. You would more then likely get a huge bad algae growth. If your doing enough water changes and/or adding any mineral supplements there should be enough iron and whatever else they want in the water anyway.