You want these "titan turd" vermin to spawn from titans? Like the RNG titans? Youd need dfhack to do both of those, because 1; i dont know of a way to spawn vermin without at least a reaction, and npcs in either mode of play wont perform reactions on their own whim, and 2) you cant modify the RNG titans at all without dfhack, so youd end up having to write your own titans and set the rng made ones to 0.
Theres the grasstrample tag that destroys a % of tiles with grass on them that the creature steps on, but i dont have a clue if that would work on vermin. It would only leave behind soil tiles, because without grass the tile reverts to its normal state, and grass cant grow on boulders or stone piles. And actually im not sure if vermin can be immobile too, they dont follow the movement rules real creatures do.
To actively change tile types accordingly would require dfhack too. To do this you wont just want to write a functioning "pile of shit" vermin but youll have to write a bunch of dfhack scripts to make them show up around titans, turn soil tiles into rock etc.
And keep in mind, creatures dont actually do anything off map, so titans wont go around turning everything to stone with their shit except when youre in the area with them/they attack your fort.
I could help you as you go as far as vermin go. You could start with the cave blob creature as an archetype, get rid of the materials and tissues its made of and borrow the tissue insert from the bronze colossus, and just change the "inorganic:bronze" to "filth_b:none" iirc. Check the wiki on that. Then youve got a single part blob shaped critter made of nothing but shit, and all you have to do is make sure you change all the names, remove its biome spawn info, and make it a vermin, and all the vermin tags are available on the
creature tokens page of the wiki. Everything else youll need for the vermin critter should be there too.
The dfhack part, not a damn clue. I dont touch that stuff with a ten foot pole anymore, i always just broke things and made the game crash or just couldnt figure out how to get from a to b. Like actual scripting takes effort, and im lazy.