Help with ID on Black swirl

[604reefer]

I'm not sure what this is... it's been growing slowly on the mushroom rock. There are also 3 of these on my frogspawn, but 2 look clear - the same color as the frogspawn skeleton, the other is black. I think there's also one on my hammer plug base, but I'd have to turn it to get a good look. Tank is 4 months old.

Any ideas? Googling "black swirl on coral" and variations haven't yielded me much. I took a look through http://www.lionfishlair.com/hitchhikers-guide/ and it could be a "peanut worm" or "vermetid snail" but I don't know why 2 are clear and one is black on the frogspawn? Thoughts?