Red growth

I have curious growths in my 75 gallon tank. They grow to about a 1/4
to 3/8 inch diameter, dark on the circumference and lighter in the
middle. So far they only appear on the two ends just at the boundary
of gravel and water. It has been a couple of years since the first
ones showed up. I removed the 2 I could see with a razor blade, but
they have come back and the tank has 5 or 6 on each end now.

Do they pose a threat? Any one know what they are?

dick