Did you try vac'ing to filter? And did the pressure still not rise then? Did you see any of it blowing back into the pool through the returns? If you vacuumed to filter, and either saw it blowing through the returns or the pressure did not rise, then you may have a problem. A 1.5 HP pump is awfully big for that pool, and I suspect it is just blowing the stuff back through the filter instead of allowing the filter to catch it to remove it from the pool.

When you turned off the pump, did ANY of it settle to the bottom, or did it look equally as cloudy as when you turned it off?