BJS,
Based on what you've mentioned I think you may have a couple different things going on. It sounds as if you're just beginning your ammonia/nitrite/nitrate cycle. Adding more rock to your tank may have, if it had any die-off from when you bought it to when you put it in your tank, delayed/restarted the cycle.
The coral you mentioned could be another issue. It's generally accepted that it's not a good idea to put "wild harvested" pieces like that into a sw tank. The problem is one never knows what, if any, contaminates the piece could have absorbed in the ocean. The water volume in the ocean mitigates toxcicity but since your tank is so much smaller, minor amounts of contaminants can have a deadly effect. You mentioned that you cleaned the coral. Boiling the piece for a few minutes is the only cleaning process I've ever seen recommended. If you didn't do that, I'd remove the coral immediately. If you did boil it, I guess it's your choice to leave it or not.
Jim