All rock and decorations in your tank will get an accumulation of detritus and algae on them, it's only natural.
If you want to clean it, remove the rock from the tank (check it for any fish that might be inside the holes) and place it in a clean pail. Add water & 1/4 Cup of unscented bleach and allow to soak overnight. Dump the water out and scrub the rocks if you want. Rinse the rocks extremely well with fresh tap water to flush most of the bleach away. Put the rocks back into the pail, cover with fresh water and double the amount of dechlorinating product for your 5 gallon pail. Let soak for an hour.
You should not be able to smell any bleach left on the rocks before putting them back into your tank.