Remove the rock, and put in a diluted solution of bleach water in a bucket. Let it sit for about ten minutes, then use an old toothbrush to scrub the entire rock while it's in the bleach water. After this you should notice NO algae remaining on the rock, if you do scrub again. Dump the bleach water and make a strong solution of dechlor and water (three times what you would use to treat pwc water - I use five times). Resoak the rock and again scrub the entire stone with the same toothbrush while it is still in the bucket. Let it sit for about ten minutes then test the bucket water for any remaining chlorine, if zero pull the rock and make one more solution of dechlor and water (same concentration as you would do with a pwc). Soak for ten more minutes then put back in tank.
This seems like a long process but I have done this sucessfully many times with my whitish rock as it grows algae on it and needs to be cleaned every other month.
I have kept otos in my tank for at least the past 8 months. After putting in the first four, two died after the first couple of days, the remaining two lasted about two months. I got three more after that, and they have been happy ever since. For those I took more time acclimating them to my water, as I have heard that they are sensitive little suckers. I know that they do a job cleaning the tank because when the first four were gone, the tank started growing algae quickly. Remember that even though they eat algae, you should still feed them some algae tablets; I do a tablet twice a week.
HTH
