From what I read Muc Off is meant for the frame and other external parts, not for lubrication or internal parts. In addition you may have sprayed contamination INTO the housing. You need to properly clean things out. If the housing stop on your down tube has a split you can use that to get access to the area of the cable that is covered by housing to clean and lube it. Shift to the largest cog in back, and then without turning the cranks shift the lever all the way back to the small cog position. The shift cable will then be slack. Pull the housing away from the down tube stop (see pic below - not at the down tube but same idea) and you will then be able to do the same with other sections enough to expose the cable, wipe it off, spray a light lube into the housing, and get things cleaned/lubed. If you see any corrosion replace the cable. When done, put all of the housing back in place and shift through the gears a few times to check things out.