It is quite simple, when you put the size of the Universe into perspective, there is literally no chance that there is no other life elsewhere in the Universe.

It is estimated that there are between 100 and 200 billion galaxies in the Universe, possibly even more. On average, there are over 500 million stars per galaxy. The chances are, due to the nature of which stars are formed, it is extremely likely that every single star has at least one planet around it. Therefore the number of planets in the Universe is a much higher number than the theoretical probability of life beginning.

1. This bacteria found on Mars is not proven to be alive, and therefore may not be a so called "Alien". However if this bacteria is actually proven to be alive, this is most certainly considered to be an "Alien" bacteria. The idea of finding alien bacteria on Mars, a planet so close to home, would give further evidence to show that it is simply not possible for us to be the only developed life form in the Universe.

2. Why not? If on the planet they evolved on, there may have been an advantage for them to have black squinty eyes, or green coloured skin. There is simply no way to discredit the theory of the "greys" just because of the way they look. I am personally extremely skeptical of advanced alien life having visited Earth, but this does not mean a life form somewhere out in the Universe could resemble the so called "greys".

I really don't think you would expect me to deny that there are other life forms on this planet when this is my username

Of course we are not alone. If the Universe is ever expanding there are an infinite number of planets and at least another other than ours has life even if it is only at it's day one stage.