One of the places the "goo" can come from is your fill water. Biguinide does not react well with chlorine and if there is a free chlorine residual in your fill water ( as there is in most city water) then it will react with the biguinide and create the "goo" which will clog up your filter and deposit all over your spa.
the white water line sounds like scale deposit. that will happen with any type of sanitizer if your water is not balanced. Get a drop based test kit and check your Alkalinity, Calcium (even if you don't add any there might be calcium in your fill water), and pH. Test strips are not accurate enought to properly balance your water.
I also noticed someone comment about the 'musty smell' of a biguinide sanitized spa in this thread. That can happen if the hydrogen peroxide oxidizer is not used enough and there is a high level of organics in the water. It means you need to shock (this is assuming the biguinide levels are proper)
AS far as the blue light cover deteriorating, that is most likely from your ozonator. Anyone who has ever used ozone in a salt water aquarium (and it's been used in aquariums MUCH longer than it has been used in spas) will tell you that ozone will degrade and destroy plastic of variuos types!
Hope this helps!
Degrade the plastic! Wow. Sounds like there are a lot of problems with ozonators. Can they cause an odor? I'm reading they eat up chlorine? I have a "Sunzone Ozone Water Purification" system (6472-625) in a 97 Sundance Spa. Do they wear out and then cause a problem? I see the bubbles coming in. Does that mean its still working? Or does age mean one way or the other for working correctly?