Clownfish maybe sick?

[TGIF]

Hi, I noticed today that my clarkii clownfish's tails have black on them now. Before they were yellow/white.

It kind of looks like streaks of dirt. They do not have any black spots on them, are eating well, swimming lots, no gasping at the surface. They are flashing up against rocks occasionally. Actually they seem to rub on the rocks to loosen up algae then eat it.

Is there any reason why their tails would change color? I've only had them 2 weeks so don't know much about them yet.

thanks

 

[Rhizo]

Possibility of ich?

 

[TGIF]

I'm not sure. They dont have any patches or slime or anything on them. They look great otherwise and are growing and active.