I don't think there's one unified Marxist position on most things tbh, since sure Marx had his ideas but lots of people in his time and since have adapted his ideas based on their own different material conditions. Unless you mean specifically "what did Marx say about this" but as a materialist even he would want people to re evaluate what he said because we learn more and things change over time.

I'm not aware of him writing about aliens though at all so. Probably there isn't one.

Now, do aliens exist in the universe somewhere? It's statistically very likely. Have they come to earth? That's almost a statistical impossibility.