It looks like some kind of sponge, rather than an algae.
Not all sponges feel/look like an actual sponge, like the typical pineapple sponges do, some slime up on the outside to control flow through their internal structure.
Maybe it's a Hymedesmia type? There's several types but some are dark like yours, other similar species are bright yellow (I have this stuff) and bright pink (I have this stuff too).
If you're keen on getting rid of it, it's likely a sign of high nutrients and/or excess particulates in the water column. In short, starve it out with less feeding, more water changes and a filter sock/finer filter sock if you already run one.
It won't likely continue to proliferate and take over the entire tank, I've had small patches that came in on the rock that haven't gotten any bigger at all. I've had other bits appear of out nowhere too, but grow to a size and remain that way.
They usually appear in low flow, low lighting areas too...