Pretty sure this is cyanobacteria, thoughts?

Try to remove it,
If it is easy to remove and slimy and regrow practically overnight, then it is blue green algae.
But I doubt that as bga normally found on substrate or plants.

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By the way, no need to do blackout unless it's an algae bloom

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I've never seen BGA grow on rocks, but maybe that's just me. To confirm what it is, siphon it out and take a sniff. BGA gives off a noticeable smell you cannot miss (something like mud smell).

I smelled some of it last night and it did smell kind of earthy, from what I had read previously it was my understanding BGA was supposed to smell super rancid, however mud would be a more fitting word for what my impression of it was. It did however look to be getting worse today after trying to spot treat it with excel, so I decided to stop watching it grow and took the rock out and boiled it for 30-ish minutes... fingers crossed it doesn't come back. :|