Type of Algae?

[Lakelander]

Does anybody know what that is? I spotted it on the shores of a large lake, on top of sand (an artificially created beach). Thought it was a some sort of algae, but wasn’t too sure, hopefully someone will enlighten me.

 

[Disaronno]

I think that's iron-oxidizing bacteria.

 

[PARAGUAY]

I have seen something like that were a farmers land drains reach a path,