I would recommend not feeding phytoplankton to your rock. It will not do anything but add nutrients to your tank and cause algae problems. Your live rock will do fine on it's own. It takes time for the sponges and coraline to grow but it will come. If anything does with a 2-part ESV B-ionic solution. That will help with alkalinity and calcium levels in the tank and won't do anything to your nutrient load. If anything it will slightly reduce the impact of phosphates in your tank.
In my experience, corals need light and a decent amount of it. Unless you get a coral that need regular feeding (which I advise against unless you really want to take the time to understand and learn about it) light will provide all the energy you need. Spend your money on good lights, it is your tanks sun, the provider of energy, which makes everything happen in your tank.