Its normal, its airborne contaminents from your DIY CO2.
You need to add a gas separator betwen the DIY chamber and the diffuser.
a 20oz mt. dew bottle works great. Airline from DIY chamber comes in, runs to the bottom. second airline exits the top and goes to the diffuser. Fill halfway with water, so the second airline is NOT submerged. The co2 rises to the top and escapes...the contaminents stay in the water.
no more white snot on the diffuser.