please tell me what is this,i clean it out and it comes back.
It could be an algae or a fungus or even sugar from a soft driftwood or a fermentation type of carbon dioxide system or a piece of fruit. Algae and fungus all need nutrients to grow, so that's a good place to start looking. Uneaten food is the most common source. In addition, too many fish for the aquarium can provide excess nutrients through respiratory and digestive wastes. Regular, partial water changes will compensate somewhat.
If the problem is algae, the aquarium could be receiving too much light. It should not be exposed to direct sunlight, and you can try operating the aquarium light between eight and ten hours a day. A fast growing, floating plant will also use up excess nutrients and provide some shade.