i see... in my tank i use tetra filter and it has water flow i guess (pic my filter link below)....i dont think that is dirt.....it keep coming even after i try clean the plant with water....if alga how can i solve it......it is possible if the cause by sun light or size of tank...

http://www.angelfish.jp/img/Tetra_On...terOT-30in.jpg




now i removed all all that get that problem and only put the plant that like grass on the pic above.....the and the problem is solved....but why only it happen to the red plant and other plant an previous pic...