if you begin maintaining calcium (and other trace element) levels and keep your light bulbs fresh, and introduce some coraline covered rocks and seed your tank (brush scrubbing the coraline in the tank) it should spread. keep in mind for good coraline growth you have to maintain really low phosphate levels as phosphate is leathal to coraline, the rocks will then color up on their own.
this of course is just an observation of possible route as I know nothing about your tank except the picture.
by the lack of anything growing on the glass I am guessing the tank is relativly young.