The most reliable way to tell if it's algae is to measure your chlorine at night, and again in the morning, and compare the two readings. If you've lost more than 1 ppm of chlorine during the night, then you probably have algae. It's hard for me to tell from looking at your pics, but if you have brown stuff blowing from the returns I'm betting that you have sand blowing back into the pool, not algae. This can happen if the laterals are damaged, if your pump is oversized for your filter (take a look at that label on the side of the motor and see what horsepower it says it is), if you've gone too long without backwashing, or if your multiport gasket needs replacing. Do you have a working pressure gauge on your filter? Does it rise when the pump is running? When is the last time you backwashed the filter? After you backwash, do you run it for a minute or so in "rinse" mode before going back to "filter"?

Janet