you could kill the algae off slowly over time by starving it of nutrients, but that rock looks very dense and boulder-ish to me. i would probably get new rock. for a tank that small, i would want to hand pick the rock myself and only use the most porous pieces with some character.
you don't have to wash out the tank- just get the new rock and let it cycle.
(turn the lights off too...you don't need those on unless you want the algae to grow more).