Nontoxic things aren't poisonous. It's generally safe to eat or breathe nontoxic substances, and they don't harm the environment.
It's a good thing if the ingredients in your dish soap are nontoxic — it means the suds washing down the drain won't be harmful to the environment, and the residue left on your dishes won't hurt you, either. You can use the adjective nontoxic to distinguish something from its more dangerous, toxic variation. Toxic waste poisons the groundwater or makes nearby animals sick, but nontoxic waste tends to break down without any negative consequences.