How bad is your snail population? If you have hundreds of bladder snails running around your tank, you could actually be seeing their poo all over stuff. I've got 3 adult MTS(lots of itty bitty babies) and two large Nerites in one of my 10 gallon tanks at home, their poo is everywhere and very noticeable. The other option is that it is in fact a diatomaceous algae forming and coating things. Try adding carbon or several Purigen pouches to your canisters, this will reduce silicate and organic material entering the water column.
A working pic would be very helpful though to conclusively say anything.