It doesn't have to be FTL capable after the crash. Just imagine, you with a rifle, an armor, a good sword and some grenades went back in time to say 20K yrs ago, for sure you are like God to the primitives for a moment but whence you exhausted the resources it's done business.
Same thing for aliens, imagine an alien refugee ship capable of FTL but needs a special fuel that it has to collect from deep Earth ocean very slowly, perhaps for a few million years because Earth only has very very few of such fuel. There is not much the refugees can do. They don't have weapons, they still habe to gather food and energy, they can't do a lot of bad things to locals (us)
That’s dumb sci fi logic though. Aliens don’t need deep ocean FTL fuel. If you can move matter at the speed of light, you have the capability to kill everything on earth very easily.
Same thing for aliens, imagine an alien refugee ship capable of FTL but needs a special fuel that it has to collect from deep Earth ocean very slowly, perhaps for a few million years because Earth only has very very few of such fuel. There is not much the refugees can do. They don't have weapons, they still habe to gather food and energy, they can't do a lot of bad things to locals (us)