Santa Monica Airport is right next to the Pacific Ocean. You have on shore flow and high relative humidity, every single morning, as a result, hence foggy and low clouds on many mornings.
Your fuel tank cap is vented. There's your sign. Your fuel tank breathes air, every single day and night, with temperature changes. When it does, water condenses on cold bare surfaces. Don't provide any cold bare metal surfaces, always store with a completely full tank of fuel, clear up the filler neck. The only thing that should be in contact with metal surfaces inside your fuel tank is fuel, not air, during storage.
Always top your tank off with fuel before storing in the hangar, and you will minimize the problem. Premium MOGAS that's E-10 will absorb quite a bit of moisture and pass harmlessly through the engine, but you MUST fly 1x per week and burn 5 to 7 or 8 gallons every flight, to create fuel turn over so it doesn't become problematic using mixed e-10 with 100LL. Your Rotax is actually designed and engineered to run completely on Premium Mogas, not 100LL. Either that or you are buying "wet" gas from airports close to the ocean, all the time. You fly to CMR a lot, it's close to the ocean, could be wet gas there, too. Try doing a few full fillups at KSZP, inland @ $4.15 a gallon for 100LL currently, then top off with 4 or 5 gallons of premium Mogas back at the hangar at KSMA. Due to fuel prices, KSZP has a LOT of high fuel turnover. We've never once gotten a tank of wet fuel there in the past 13 + months. Gascolator always shows just faint blue tint of fuel, never even a drop of water.
Just a fact when you run fuel with ethanol. You need to have regular fuel turnover to prevent problems.