Some background info first.
I am in a brand new home, my water heater is a Bradford White 50 gallon.
It has a powervent installed. The 2" into 3" PVC vent pipe goes up about 2 feet and then horizontally for about 15 feet where it vents out of the basement wall.
I have a 72" by 48" fiberglass tub in white.

I have the unit set above the normal heat to a hotter setting, but not the highest available.

We are on City water not a well.

I have noticed little black specks on the bathtub recently. There is no shower in the tub. They were not there for the first three months of use. When rubbed they spread like ladies mascara would spread. It takes some rubbing and cleaning to get rid of them.
I thought at first maybe it was some kind of mold, but since it was only inside the tub, I decided to clean it with a 50/50 bleach water mix.
I let the tub sit for 3 days and nothing else appeared.
But once I filled the tub again with hot water, the little specks appeared again.
They are not in the sinks or toilets.
The toilets "do" show a very light pink colored ring at water level if not flushed for a couple of days, which cleans off very easily.

Does anyone have any idea what this is called, what causes this and how I can get rid of it.
All help would be appreciated.
If you need more info please ley me know.