Why are there white deposits on the surface?
White deposits are caused by lime or other minerals in your water. This will also cause the pH to become very high. Plants will not be able to take up nutrients in these conditions. To resolve this you should consider using a reverse osmosis carbon based filter to clarify your water source. To remove existing deposits use distilled white vinegar in a spray bottle to dissolve the white deposits. The acidic vinegar will also lower the pH in your system.
What can I do to prevent fungus?
Excessive Moisture or Decay
Moisture and low light conditions along with natural decay of plant materials in the pockets make the perfect conditions for fungus and/or molds to grow on the surface of the felt. Often this is due to excessive moisture so also try to dial back the watering times.
Clean Up With a Light Bleach Solution
To clean up fungus growing on the felt you can use a very light chlorine bleach solution in a spray bottle. Add a about tablespoon of bleach to a quart of water. Then spray the affected areas. The bleach will quickly dissipate so as not to harm the plants. And the bleach will not affect the color of the felt.
Bleaching of the Florafelt pockets and wraps is also a good way to clear a used panel when you want to start fresh.
Neem Oil Concentrate
Neem Oil Extract is also a great way to prevent fungus from forming on the felt surface. See: Controlling Pests Naturally