Would that happen to be the latest Cain novel; the one involving the Tau? I have yet ot read it but I am interested to see how it plays out. I don't remember reading of it in Warriors of Ultrmar, but I will double check as it has been some time since I read the book.Cain novels shows this as well.
In any case I just cannot see the Nids leaving anything other than rocks and earth behind. They have consumed quite a few opponents and created quite a few special creatures to adapt to the ever-chaning battlefield.
For example the "Doom of Malan'tai" from HF Medusa, a Zoan that was adapted to extreme proportions and even was noted for having gorwn a natural "Hood," to help it feed on the souls of it's prey. this was probably from the high number of Imperial psyker the Hive Mind ingested and put to good use. "Old One Eye" from HF Behemoth that is siad to continue to ravange Calth to this very day. It's ability to regenerate itself even after having taken mortal wounds is something that is akin to a Space Marines ability to regenerate from taking normal wounds that would kill any normal human. Perhaps the "Red Terror" perhaps, which was adapted to fight in the tunnels of the mining world of Delvan. I'm sure the Hive Mind had to absorb some minerals, or mining equiptment, and deduced that sucha creature would make the perfect killing machine.
Just saying, I cannot see the Nid leaving anything of Biomass, or genetic value on a planet. But I suppose I'll end this here as it's more my perspective at this point less of an actual debate.