Black Spots on Rock?

[Humuhumununu]

I have had a sustainable environment in a 135 gallon aquarium for about a month now, and today when I was making my regular observations, I noticed some black spots on the rocks. Now I have completed my cycle, the hermit crabs, snails, and Foxface are doing a good job eating the algae, and some of my hermit crabs have purple shells ( which I have always asssumed to be Coraline Algae). So my question to you guys is are these black spots the beginnings of Coraline Algae?

 

[little_fish]

without a picture its impossible to tell you anything about the black spots, it could be a million different things

 

[Bifferwine]

What does it feel like? Hard or soft? Black sponge is somewhat common and appears as spots in the beginning -- but it's soft and velvety to the touch.