Is this cyanobacteria?

[Simsung]

Hello

I have a crimson coloured growth on my rockwork.
I thought it's coralline algae so I let it be but as it grows I am not so sure anymore.
Colour on the photo is 95% true to life


There's absolutely nothing growing on the sand or in other areas, it's just the rockwork here and there in spots like this. It's showing in the highest flow areas.

I've taken the rock out today and tried to scrape it off with a toothbrush but it didn't budge.
I tried taking it off with a fingernail - it's not coming off. It seems to be hard-bodied and is slightly slick to touch.

I have corraline algae on other rocks but that is a lot lighter in colour.

 

[cshouston]

It’s coralline, especially if it doesn’t come off with a toothbrush. Cyano is very slimy and becomes stringy.