Dark spots? Id

[OutsideBrian]

I’ve noticed some dark spots starting to grow on my rocks. Any ideas on what it may be?

 

[Flippers4pups]

Those look to be sponges.

 

[OutsideBrian]

Those look to be sponges.

They’re super thin kind of like coraline.

 

[shopan]

I also having this dark encrusting spots growing. Noticed alot after increasing the light. Tank is about 5 months old and rock is covered in green coraline. Hopefully someone can help ID this.