I have heard that using distilled water would help that. It could be due to the minerals in the tap water if you are using the isomalt that needs to be prepared. If you are using the isomalt sticks I think that they are a melt and pour and they don't need to be heated to the hard crack stage.
It sounds like you're not using distilled water. Water from faucets have chemicals in them which will prevent your isomalt from being clear. Also, it could be the kind of pan you're using. Are you using one with a non-stick coating to it?