How to clean silicone ice cube forms

I bought silicone ice cube forms and when I used them something white swam on the water. I guess some silicone that dissolved from the form.

First I made ice three times and threw it away, but the problem persisted. Then I tried to put them in the dish washer. Now I bought another product from a different manufacturer and the problem is even worse.

(I am NOT 100% sure if they may still be products from the same factory or made in a similar way, but I explicitly tried to buy another product)

Are silicone forms just crap, am I doing something wrong, or is there a good way to clean them before the first use?

I do not have the problem using the same tap water with the hard plastic ice cube maker that came with my fridge. The general water quality here is quite high and I even use the water with a sodastream and making tea in the same glasses and it looks perfectly clear.

Before freezing the water looks clear as well an I do not see such dirt even when I let a glass of water stand somewhere.

Update:
I made an experiment by freezing one form with and one without lid. Here is an image of the forms and the lid:

The blue one was used without lid. They seem tp be both silicone and the lids are all the same, only the forms have different colors.

The result after dissolving all cubes looks like this:

Left: blue form without lid (Little dirt, but still too much)
Right: white form with lid (Larges flakes of some dirt)

The cubes in the open form were quite easy to remove and well-separated. The lid stuck a bit on the cubes and I needed to break the cubes apart by bending the silicone, because the lid hold the water surface flat.

The open form stood on the closed one.

Update 2: Boiled water has the same problem. I'll try a water filter next.

Update 3: Water hardness as published by my water supplier:

• Hardness 4,7 – 8,1 °dH (soft)
• Magnesium 5,2 – 9,0 mg/l
• Calcium 24,6 – 43,5 mg/l
• Natrium 11,3 – 21,9 mg/l 200 mg/l
• Chlorid 23 – 39 mg/l 250 mg/l
• Nitrat 10 – 21 mg/l 50 mg/l
• Uran < 0,0002 mg/l 0,01 mg/l

Update 4:
Same result with the water in another city (unknown hardness). In some of the forms white sediments after the water rest dried. Next I try a water filter.

Update 5:

I tried a water filter and got the same result.

Using a water filter I still get white flakes like in the right glass after melting all 24 ice cubes of the new form (not the one pictured above, but also silicone) with lid.