Strange growth on rock

[SuncrestReef]

Over the past month I've noticed a dark red patch growing on my rock, and now it has developed a brighter red stripe. It does not appear to be coraline, or at least it doesn't match any of the coraline I already have in my tank. Any ideas what it might be?

Here's a macro photo of it. The specimen is about 1/4" wide.

 

[KJ]

It is hard or soft?

 

[SuncrestReef]

It is hard or soft?

I haven’t touched it yet, so not sure. I’ll check on that later.

 

[SuncrestReef]

It's hard, but it does have a bit of a slippery/slimy feel on the surface.

 

[taricha]

Oh, that's a fun one!
This is coralline algae. Some kinds of coralline turn orange when dying, (the pigments break down at different rates). Something went or was scraped across it and killed a striped path.

 

[TriggerFinger]

The lines make it look like a bivalve. Have you ever seen it open up?

 

[Tonyreeffish]

It almost looks artificial. Did you get the rock with the dark spot on it? The algae could have died off revealing some kind of artificial bond or cement. Quite odd whatever it is.