Hi Wingless;

1) You can 'DE' test your filter, to see if there's a problem.

Purchase a small amount of DE (or a large amount -- it's not very expensive) and add a coffee can full (or equivalent) to the skimmer while the pump is on and running. Then check your hi-flow eyeball, to see if a cloud of DE returns to the pool (it shouldn't). If it does, your filter has problems. In the case of a cartridge filter, the most likely problem is that your cartridge needs to be replaced. After wards -- if your filter is OK -- you can add small amounts of DE to improve filtration, but it will also greatly shorten filter runs.

2) It could indicate problems, or it could indicate that someone has tried to balance pool flow by putting flow restricting eyeballs in the nearer units.

3) I don't like to encourage people to swim when they can't clearly see the bottom -- for lifeguarding-type reasons. But dead algae, IF you also have a decent chlorine level, is not a health issue.

Ben