I only clean my front glass, i let the snails have the rest. Lots of algae is unsightly, but it is a natural thing for a tank to have a little. I wouldn't scrub the cave off unless it was a type of algae that nothing i have would eat, or if it was just growing out of control. I find in my tank that my snail population rises and falls with the algae, so if there are blooms on surfaces, i only see them for a few days until the snails catch up to it. Can you describe your algae? Some types are more desireable than others.
Limelight - it is the direct sun causing the algae in that spot. Try to prevent the sun from reaching the tank if possible, and/or be diligent about scrubbing that spot of the glass to keep it under control. Make sure you use a special scrubber designed for acrylic tanks if your tank is acrylic. Glass tanks you can use one of the green scotch scrubbie pads, just make sure it has never been used for another purpose. The green spot algae is very hard, it will take some elbow grease to clean it off, that's why your plec wasn't helping.