Man you should have leave the coral a lone and take the rock out and give it a good scub with a good strong brush to get rid of all the algae. Do the same to all the rocks. vac your substrate under the rock and every where. Do large water change. If you think your coral is not use to the MH light, get the light defuser from home depot and place across your thank for a few week. Bring your Mg/cal/alk/ph up at or higher than the recommended level. Throw the Phospate/nitrate test kit in the garbage. (In a relatively mature reef tank you, have phospate/nitrate but will read zero because it consume by algae and by the coral) regular cleaning will keep them down. Keep the Mg/cal/alk/ph high will keep the phospate from leaching back into your water from gravel and rock. Too bad with the coral good luck.