Re: we just can't see them for some reason
AIUI the thinking is there might be things we don't realise are alive. Desert varnish was one example, though the conclusion now seems to be it's due to chemical processes. When there are weird things like snottites and stromatolites hanging about it's easy to miss them. One problem space agencies have is working out what life might look like on other planets, and a related question is why there isn't any non-DNA/RNA based life on Earth (or whether there isn't, as our most sensitive methods of detecting microorganisms look for RNA). It's also thought there may be a lot of microbial life in deep crust, so a lot of places we haven't looked.
We are talking about microorganisms here, rather than things with tentacles and three eyes, and if there is any to be found then it may well only be alien in the 'different origin' rather than 'extra-terrestrial' sense. But we certainly haven't checked everywhere yet.
There's some weird stuff that's very terrestrial, xenophyophores are one of the most common animals on the abyssal plain. They are large single-celled organisms, essentially amoeba, but up to 20cm in size.