what i would do is remove the rock and siphon out as much of the waste as you can. How I am going to set up my refugium is have a section that holds live rock rubble for frags of course and have the overflow water enter in that section and go through the rubble and down into the refugium with caulerpa...it may also catch detritus but by the water constantly flowing through the rock i hope it will keep it flushed clean. I would find a different spot for your rubble.
Brandon