Will having a moist flower garden up against a wooden basement increase the chances of rotting the basement?
Yes. Beds should be kept a distance off of the house to insure the water doesn't soak the basement exterior walls.
Dead algae and other rotting materials will turn grey and slimy inside a filter. Chemically clean the filter and then watch to see if it reappears. Chances are your filter cleaning interval is too long giving the detritus time to break down and rot. increase the number of cleanings per season to avoid funk. Pool & Spa