The look that Mike gave Tom was one of both surprise that Geary knew about the planned casino takeover and that Tom was somehow responsible for letting the cat out of the bag. I think it was more the latter - it just reinforced Mike's belief that Tom was not all that Vito thought he was cracked up to be.

"Generosity. That was my first mistake."
"Experience must be our only guide; reason may mislead us."
"Instagram is Twitter for people who can't read."