What's this?

[LittleMissNeptune]

This popped up in my tank a few days ago, any idea of what it is?

 

[Crispii]

What am I looking at exactly?

 

[LittleMissNeptune]

Sorry, I didn't make it clear, here's a picture with the stuff circled:

It's all over the decor

 

[Crispii]

It could be mulm or detritus.

 

[LittleMissNeptune]

What is that exactly? Mulm and detritus and are they harmful?