Is it slimy? You might have cyanobacteria..and it can spread like fire. I it is caused by high levels of phosphates, silicates..The best way to get rid of it is first not to add any more water pollutants..Use RO water or better yet RO/DI water..You can buy a good filter for under 200 bucks and it will save you a lot of problems in the long run..Phosphates and silicates can also cause alot of other algae problems like hair algae ..That can take over your tank and cover everything..Also if you have fish cut back on the food as it too can contain phosphates.There are some medications to rid the tank of the cyano but IMHO that is only like applying a bandage to the wound ..It won't take care of the source of the problems and it will more than likely come back..A good skimmer will also help alot by taking the excess "stuff" out of the water. Do you have a test kit for phosphates and silicates..also nitrates..??You can test your water before and after it is put in the tank..Before adding any corals you should probably get that taken care of and add a cleaning crew to help curb andy diatom blooms or unwanted algae in the tank..
To answer you first question..After the tank has cycled and the diatom bloom is run it's course you should be able to add a few hardy corals..but make sure all your levels read zero..Ammonia, Nitrites, Nitrates..