1) You can add the Mr. Mercury effect to an adjustment layer and key frame the opacity of the adjustment layer from 100% to 0% when you want to turn it off. If you have other layers underneath that you don't want the Mercury effect applied to, you could pre-compose the adjustment layer with the layer you actually want effected. Then you could create a null in your master comp to control the opacity of the adjustment layer in your precomp, that way you don't have to go back and forth between comps to adjust timing.
2) You could simply split the layer where you want the effect to turn off, then disable the effect on the "off layer" then create another split when you want it back on and re-enable the effect. So on and so forth. (Cmd/ctrl+Shift+D to split a layer). The problem with this is you could end up with a lot of layers and unnecessarily complicated timeline. Plus if you want to shift a parameter in the Mercury effect, you will have to copy the effect and paste it to all the "on" layers, or shift them all individually.