Just turn off all filtration, and use a turkey baster to dust off the rock, allow to settle for a bit and turn back on filtration......I often dust off my reef rocks this way. Make sure turkey baster is filled with water and not air though, so your usuing the water pushed out of it to create a current to clean with. I like the long clear tube type basters so you can see if you accidently suck something up in there. For deep tanks you can take a piece of the 3/16" diam hard plastic air line sold at petshops, and slip a short 1/2" long piece of regular air line over it, so the air line is flush with the end of hard plastic airline tube. Pull off the rubber bulb end of baster, slip the ridgid hard plastic airline into baster tube, until the piece hits the bottom of tube, and wedges against the piece of airline you installed on the hard line......(may need to open hole up slightly or add a larger l;iece of tube to act as a stop. Replace rubber bulb and cut hard line to length you need......Now its extended and you do not need to have your hands in the water to use it......The soft airline on the hard line acts as a stop and a seal.....