The likelihood of intelligent life from another planet coming to visit us in the next century is incredibly slim. If you assume that it is possible for lifeforms to travel the vast distances needed (a big assumption) it is still incredibly unlikely they would visit us.
There are an unimaginable number of galaxies, solar systems and planets out there. From our limited knowledge, a tiny fraction of planets are capable of supporting life. Fewer will have the conditions for developing intelligent life - the resources and luck needed for a species to develop over millions of years without wiping themselves out through catastrophic war or an accident or natural disaster.
Of the few that develop intergalactic or even just interstellar travel, it is tremendously unlikely that they will happen to choose our solar system and planet to visit. A bit like you and I going to the beach on separate days and both picking up the same single grain of sand.
Further, it's only been in the last 100 years or so that we'd necessarily know aliens had visited, if they had. Before radar and radio and other such things, aliens could have visited large parts of the world that were virtually uninhabited and we would be none the wiser. It's spectacularly unlikely that if aliens visited our planet they would happen to have arrived in the specific century or so when we'd have known about it, rather than at any other point in the billions of years of history.
I don't doubt that aliens choosing to visit us could be very dangerous - they would have technology so far in advance of our own that we would be reliant on their goodwill. But to my mind it would be easier for a species to develop technology to synthetically develop the resources they need (oxygen, water perhaps) than it would be for them to develop interstellar space travel.