Is my roof ok?

I was doing a roof inspection the other day and I took these pictures.

Black spots are in certain areas. Is this ok? When I run my hand across the spots there is a gritty feeling.

They have been there for a year or so, There is no leaks I just am wondering about these.....Is it time to think about re coating the roof soon?

The TT is a 2007 but it was made in 2006

Any thoughts are appreciated