Is spice worm poop? Or does it come about some other way, and the worms are attracted to it, maybe as food? Or what?
I seem to recall this explained in the books, but it’s been long time since I read them.
The worms make the spice. I don’t recall the details. I misdoubt it’s worm poop, but one never knows.
It’s worm poop.
Hey, we eat honey! 
Wasn’t it that the “nests” where they hatched their broods got imbued with spice? God, it’s been a long time since I read that book. Maybe I’d better get it out again.
If memory serves me right, Spice is actually the byproduct of the little maker’s excretions mixed with some sort of fungus. I remember the little makers as being the larval stage of the sandworm. Hundreds of these little makers can be found in nests hundreds of feet below the surface. When they grow to a certain size, they excrete a great amount of water and refuse, and then move on to become sandworms. Fungus feeds on the excretions, creating a whole lot of gases. These gases eventually get released with great violence in a “spice blow”, dragging up with it all of what is left of those excretions and depositing it on the desert floor. The excretions dry up and become the greatest commodity in the universe.
This is all from memory, of course.
It’s not exactly worm poop. It’s kinda complicated, but I’ll try. Sandtrout are larval worms, unicellular organisms, and are water scavengers. They join their bodies together and transport water to their nest site. The excretions of the worms mix with the water to form a pre-spice mass. The mass builds up pressure and then explodes out onto the surface; when exposed to air and sun, the mass would become melange. Many sandtrout are killed in the explosion, and their bodies are also a key component in melange.
The above was taken largely from The Dune Encyclopedia, by Dr. Willis McNelly.
That’s complicated. No wonder I keep forgetting it.
I thought the sandtrout came out of a sandword corpus (asexual reproduction). The sandtrout eat the body from the inside, and the sandtrout excrement mixes with the rotting sandworm to create a funky fertilizer bomb, the pre-spice mass.