Not 100% sure unless you have a picture. But it sound like it. If you wait longer, you might see mold growing on it. Perhaps you may collect those out now and isolate them.
I think there will be problem if feeding artemia, since they will dead after a whilein fresh water. And I guess your tank will be big, too big for the newt larvae. So unless you feed a lot of artemia, otherwise, the larvae wont be able to eat them enough. Since at early stage they will move very little.
And since the artemia died fast in fresh water, the water quality will be lowered very quickly and will be difficult to siphon out the waste without siphoning out the larvae.
Just my opinion.