If it is one rock that is easily removed I would get a (clean) bucket and put some tank water in it. Get a new cheap toothbrush or similar and just scrub it off using the water in the bucket. Have a cup ready to get some water from the tank to do the final rinse in the bucket before placing it back in the tank. If taking the rock out is no good then get a siphon hose and a toothbrush and do the best you can in the tank. You need to stop the source of excess nutrients etc. or it will just come back. Most of the time it is from too much feeding.