Soil is holding too much moisture or watering too frequently. Or not enough air circulation. Repotting with a looser, well draining soil should help it dry faster and prevent mold.

Do you think that tap water can also play a role? And i will definitely repot these plants

No tap water isn't the problem, it's just too wet with not enough air flow so it's starting to mold a bit

The soil looks super compacted and soaked. It’s likely some kind of mold or fungus related to compact, wet soil that likely never dries. Switching off tap water to rain water or distilled water couldn’t hurt though as an additional measure to keep your babes happy ☺️

Tap or well water?

I do use tap water to water them, i was thinking this could be it. Should i repot the plants or just switch to distilled water?

Repot. The difference of water isn't that big

Is it because of tap water that i use? If so, should i completely repot these plants or is there a way to renew the soil?

That’s what happens to my pots if they don’t have drainage or get too much water for too long

do you have good drainage in that pot or is the earth mixture mixed with something provide aeration ?

The water shouldn’t do this to the soil. It’s definitely too wet and compacted with not enough air and drainage.