Hey yea I found one of those too on my glass but it comes and goes. Sometimes its there and sometimes its nowhere to be found. I thought it was like a slug or something.
Is it flat? or rounded like a snail? If it is rounded, then touch the top of it and see if it retracts to reveal a shell, if so, then it is a cowry snail, if it is flat, then it is most likely either a polyclad flatworm or a slug of some type. You may be able to get an ID on it by going through the snail and worm sections of my hitch hiker pages. Hope it is of use to you.
Are both images of the same animal? I can't tell what the second one is but the first one is a flatworm. You can see the mouth area & internal organs showing white through the transparent skin. As nearly all flatworms are predators on other organisms you may not want to keep it.
i think those are ordinary snails/slugs. they have a shell, but it's not pronounced like a snail shell. i have them all over the fuge and some in the tank. if it's the same, and mine look very much like that, all they do is eat algae. no problemo