For one thing, we don't know everything about the aliens, their abilities and how they sense things. If you want a simple answer, the alien could sense some queen pheromones emanating from Ripley.
As for Clemens, it could be either reason or both. I doubt if the alien knew what a syringe was, but might have perceived it as a threat. Or, it could very well after a new victim.
When the alien "came out of the corner", it saw Dillon as a threat to Ripley.