Tyranids don't eat necrodermis:

• Shield of Baal [Devourer]:

"The mindless warriors retreated in slow lockstep. Many were lost beneath the slavering claws of the tyranids, pulled down and gnawed upon by mindless beasts that were yet to realise there was no bio-matter to be had from them. The ripped-fabric sound of gauss weaponry grew ragged and inconstant."

• Shield of Baal [Word of the Silent King]:

"Our cold bodies hold little interest for the Devourer. At best, they might be drawn to the more physical power sources utilised by our technologies, or defend themselves when we strike them. But fodder for their living ships, we are not."