How to clean silicone ice cube forms?
Silicone does not dissolve in water.
The scum you see floating on the top looks very much like lime-scale. This is in your water supply. It will also give you an encrusted kettle &amp; white/grey blobs stuck to the nozzle of the hot tap after some time.
You could try distilled water, or invest in some kind of hard water filtration system. As mentioned in comments, using cooled, boiled water may also help a little, as the boiling will drop some of the calcium out of the water… &amp; stick it to your kettle element instead.
Many places in the world you can enquire with your local water company as to how hard your water supply is - how much calcium, basically - &amp; they will advise on softening methods.