Sun light is not the problem! People raise corals using natural sun light all the time. Most use something to reduce the amount of light hitting their tanks, since our tanks are very shallow compared to the ocean. Look in to some sort of shade cloth. You should probably only run you lights for viewing and to add some blue if you desire. Sunlight is much more intense then any lights your likely to use. Here is a large out door system using natural sun light. >
http://www.garretts-acropolis.com If I recall he only lights a very small section with one MH. As you can see Steve used a tinted cover to reduce the amount of sun light hitting his corals. He also has a very large sump underneath full of live rock and macro algaes.
I have a 300 gallon tank full of cured live rock outside. I quarantined my fish in for around 9 months in it before I set my reef tank and I did not have any algae issues. I does grow cheeto well.
All reefs have algae. Herbavours keep it under control in most areas in the wild. They can definatly help in our tanks. Growing algaes in a sump can really help keep it out of the display. In your case if you can use the natural sun light in your sump it will help a lot. I have found ATSs to be much more efficient than cheeto snd other algae. If I light my ATSs 24/7 I can drive my nutrients to zero and kill my cheeto very quickly. My corals did not like zero nutrients. I cut the light vycle to my ATSs by 50% and nutrients went back up, corals are happier, macro algaes are growing again. Now I just fine tune my light schedule to get the nutrient levals I want.
As others have pointed out it is a waste of money to try to keep SPS until you know your parameters and they are stable. In addition it makes it very hard to do much more than guess at what is causing your issues.