What's it doing to him? 
Little Miss Pointy
"It seems to have a tube down his throught!"
Do you know, Ridley Scott's an interesting director? In his version, he cunningly took the character Ash and made him look more like a scientist type. He even gave him a nice English accent and neat clothes and close cropped hair. Ok, you might think - "So obvious!" I know, it seems a little hackneyed, but somehow he made it work. So did Ian Holm of course. Great actor.
NOTE: This creature is called a face hugger
Someone should tell the crew. They've obviously never seen this well-documented creature before. To be fair, I only knew it was called that because of my brief look over my late uncle's copy of HR Giger's ALIEN design book during a trip. I suppose I got a chance to show off that knowledge to my friends here!
So anyway, in this version Ripley takes the driver's seat in science matters. She even has a pointy pointing stick for pointing at sciency things. Ash shouts up to Brett - who we haven't even seen yet - to fix the blot on the x-ray. Apparently this Brett character is adept at adjusting and maintaining delicate medical equipment. That must be his job on the Nostromo. I bet he's a geeky type with glasses and a pony tail who drinks red bull all day long and fiddles with lenses and computer code. The meticulous, scrupulously clean and neat type.
Despite all this resident expertise, Ash is still clueless. Hey, now he has a pointing thing too!