“Get off my lawn” is not exactly the most enlightened policy I can think of.
The ‘new world’ would have been very much different without small pox and other European introduced diseases which wiped out huge populations before ‘white’ eyes set upon the earlier arrivals.
‘Get off my lawn’ is absolutely a valid thought when considering biologic introductions. Heck, if just they have a bacteria which thinks our intestinal bacteria is super yummy, most of us are dead within weeks.
Biologic contamination is orders of magnitude more critical, yet of course is always absent from science fantasy novels. (Oooh, don’t let them see us as we might contaminate their development...)