PDB said:

Silly question. Since I have been watching Alien so many times lately I began to wonder: does Dallas knows Ash is an android? Ash is the only person on the ship that Dallas listens to and sometimes takes orders from. When Ash says the facehugger has to go back Dallas accepts that decision in the face of everything that's happened. Dallas as the captain is the only one to have access directly to Mother so its not unreasonable to think he might know an android is aboard also. He seems to know that Ash equals the company.

I don't think so; not that Dallas couldn't have accessed the information (if Ripley had clearance once she took command of the ship, he would have had it the whole time), but that he would have had no reason to think he should. He does mention that Ash was a late addition to the crew and that he had no explanation for the replacement, but goes no further, suggesting that he just sort of shrugged and went with it.

Ash gets to make certain decisions where the science section has authority (this is not the same thing as breaking quarantine regulations, which Ash clearly did with ulterior motives).