Anaerobic bacteria?

[Nano]

I use standard brown aquarium sand in my tank and over time the bottom layers of the sand have began turning black. Even the underside of the larger rocks I use for decoration are black so I'm pretty sure it's anaerobic bacteria instead of algae.

Is anaerobic bacteria bad? I can't seem to find a clear answer.
If it is bad do you recommend just stirring up the sand or are Malaysian trumpet snails a good option? (I'm worried if I introduce snails I'll eventually have hundreds of them)

Thanks in advance for your help.

Here you can see just how black it has made the side of this rock which is buried in the sand

 

[AxolotlChris]

Anaerobic bacteria can be harmful, but often is not in concentrated amounts which can pose a problem, although, why have anaerobic bacteria when you don't need to?

How thick is your substrate? it's recommended to disturb the sand slightly where this bacteria is most likely to develop to allow the pockets of gas to disperse.

Can you take images of your tank and substrate?