No connection is ever guaranteed but you should be fine. LH currently has a minimum connection in FRA of 60 minutes so 2 hours is plenty.
The Seattle flight gets in at 9:15 in the morning most likely in Terminal Z. When arriving from the US you disembark directly into the Terminal proper without any passport or security checks.
You are most likely departing from Terminal A which is directly under Terminal Z. Following signs to "Terminal A" will lead you to an immigration checkpoint that connects the two terminals directly without leaving the secure area. This check point tends to be less busy then the main check points.
Once you have cleared immigration you are in Terminal A proper and you just head to your departure gate.
DO NOT head for "exit" or "baggage claim". Once you are in a check point line there you need to fully exit into Germany and then re-enter through security.
If you miss the connection through no fault of your own (incoming delay, massive backup at immigration), LH will rebook you on the next flight for free. There will most likely 2 more flights to Athens on the same day.