The easiest way, and this may not be what anyone would want to try - is to get very fine sand paper - it cleans it really fast and really easy. I know it may take a light film of the gelcoat off too, but when I had the pool company (from the manufacturer) come to fix some hairline cracks on the top, this is how they cleaned the scum line. I have been doing this for about 4 years and have not had any problems. I wouldn't do it too often, I usually only do it at the opening of the pool season, but it lasts most of the summer with occasional clean ups with a little baking soda and water paste. If you don't want to try this, then a Mr. Clean Magic eraser works too, but it gets used up really fast. Other ways are the baking soda and water paste, and there are some cleaners sold at pool stores for cleaning pool scum lines. Hope this helps
I've attached 4 photos of what I assume to be a scum line on my pool. I'd appreciate any comments on how to clean it. A couple inches above that there is another line that appears to be a scale formation.
By the way, it has rained a few inches here in Phoenix over the last 24 hours (rare), so the pool level is now above these lines.
If I shock the pool now, would it possibly clean the currently submerged scum(?) line? Also I may add a quart of muratic acid.
If not, is there some other chemical I could add to remove them while the water level is so high?
Salt content 3,000 ppm but chlorinator cell out of service (currently using chloring floater tabs)
One thing that I have had fairly good results with, is to clean the tile with an orange-based cleaner. Do this after your tile is clean, and the citrus will leave a little bit of oil on the tile. This seems to keep the water line from building up as quickly. It doesn't seem to work with every pool, but it does make a difference on many.
Looking at your photos as well, it looks like you do not have tile, just an aggregate finish all the way up.. (I could be looking at it wrong) If this is the case, I would stay away from anything acidic, like muriatic acid, or harsh tile cleaners that have acid in them.