So that is actually filtering the line from the fuel tank to the vacuum tank?!
If so, then that's probably normal as the vacuum tank operates - the suction is applied by the engine to the vacuum tank, the fuel is drawn from the fuel tank through the filter to the vacuum tank, and what you're seeing is the fuel arriving to, and being sucked out of the vacuum tank as the vacuum tank cycles. At idle it only has to do this every few minutes.
Roger