Black and brown spots...could it be algae?

[mizspeld]

Hi all,
I have noticed an increase in black and smudgy brown spots on the sides and bottom of my inground marcite pool. They are less than dime shaped in size. I check my pool levels once a week and everything has been in range. I've tried brushing them with a nylon brush as well as black algaecide to no avail :? Any ideas or thoughts?
Thank you!

 

[mbar]

To check if they are organic, use a trichor puck, or some chlorine in a sock and rub on the stain - if it lifts off it is organic. To check if it is mineral (metals), use some crushed up vitamin C tablets in a sock, or some ph down, and rub on stain. If it lifts off it is mineral.