Both of those questions will depend on what kind of environment they need to survive. They might need pressures higher than we can or a pH that would be corrosive to us, or exposure to an chemical that is toxic to us like oxygen was to most life that was around when photosynthesis started producing it.

If we can make physical contact with it, we'll be able to eat it. But if it is based on a different chemistry, I doubt there will be much nutritional value for us as our proteins and vitamins are all based on our chemistry. Hell, there's a good chance we won't get much nutrition from alien life based on the same chemistry because they might have evolved with a completely different set of proteins that might do things similarly but connect with each other a bit differently. There's a symmetrical set of proteins that look like mirror images of ours and would behave exactly the same, but aren't compatible with ours.