My diatoms never looked like that.
They made the sand and rock look as if it were covered by brown dust.
That looks sort of like Cyano. Although I could be wrong.
How long has the tank been cycled?
Water parameters?
Yeah diatoms are usually brown. Looks like cyano. How old is your tank and how much flow do you have? If it's a relatively new tank, cyano is a part of the tank establishment process.