The Multi FX mode in Serato DJ has preset parameters (feedback, HPF cutoff etc.) that were created here at Serato HQ for what we felt was optimal for each effect. You can't adjust these parameters for the Multi FX mode.
You can adjust the parameters in Single FX mode, but these changes won't be reflected in Multi FX mode.
For more information about using FX in Serato DJ, check out the manual here. There is also a video for how to use Single FX and other features of Serato DJ at serato.com/dj/tutorials