Basically Grid makes two kinds of spit, the usual kind and the kind xenos use to glue victims to things so their offspring can rape their faces. The second kind also doubles as a sort of terraforming shit. That gross black biomechanical shit you see in nests? They build all that with their spit. Like most xeno bodily fluids, it's mildly caustic. Plants won't grow on it, animals and insects avoid it, and if it's on your bare skin long enough it would probably cause a mild chemical burn.