take a small powerhead and "spray" your rocks down with it. Get as much of it suspended and in the water column. While it's suspended, siphon it out and do another big water change. A big clean up crew will help, they will finish off the uneaten food. A cucumber will eat detritus, as will copepods. A high rate of flow in your tank will aid in helping to keep food suspended, and uneaten food can make it down into a sump and into a filter sock. I have the bigger portion of my CUCrew in my sump, and I don't have a filter sock. That way, the uneaten food makes it down there where they can eat it. I also have it packed with live rock and there are pods galore feeding away at the detritus. Once I see a rise in trates, I can remove the lr from the sump, swirl my arm around a few times, and siphon out the nasty water.

Of course everyone has their own version of how they do it, that's just mine.
Good luck, it sounds like you have an easy situation to relieve.