This scene starts when there are orders to investigate this bird that Frux says she saw, someone tells Wrex they can't investigate, when he asks why this person says he'd better come and see for himself.
Wrex is terrified when he sees the Deadwood climbing vertically up the cliff side (in his conversation with Narse he said it will take some time for it to arrive since it needs to go around the cliff) and this is the scene when Wrex says that it knows where they are now.
The others are panicked when they see what's happening, one says "It's growing on the stone? How can it grow on the stone?" and another responds "How can it grow at all?"