Sand

[Joeyt66]

Well it could be algea it is a lil greenish looking. My lighting is 2 40 watt aqua glo bulbs (48 inches) in 2 different shop lights so it extends to my 6 ft tank. and i leave them on for 11 hours a day.

 

[Yenko]

If you do replace the substrate, get pool filter sand; it has a uniform grain size so it will not compact easily and therefore not become anerobic as quickly, which leads to the production of toxic H2S gas. Very little H2S is produced even by dirty pool filter sand, because bacteria in the oxygenated region oxidize the H2S into water and sulfer dioxide.

 

[Dubby]

Well it could be algea it is a lil greenish looking. My lighting is 2 40 watt aqua glo bulbs (48 inches) in 2 different shop lights so it extends to my 6 ft tank. and i leave them on for 11 hours a day.

It does not appear to be too much light (80W) for a 135g tank. I would suggest you reduce your lighting to less than 10hrs, if its not heavily planted.