Mescaline fumarate should be very soluble in water. I thinks is possible to precipitate excess fumaric acid when your adding fasa to limo/xylene since FA is completely insoluble in the xylene/limo. Granted adding acetone makes it slightly able to hold Excess FA by changing its polarity from it now being a binary solvent (np/alcohol), but depending on the ratio of acetone/xylene, how saturated with FA the acetone is, and how dry the fasa is, excess FA can and will precipitate.
Best way to clean them up imho, would be dissolving in 2ml distilled water per gram of fumarates and decanting off the water after 30 sec. Do this 3x, and evap the water. Discard what does not dissolve, taste it first though, it should not be at all bitter, just sour from the excess FA.
Excess FA will be much less soluble than the mescaline fumarate. Acetone is not that great at washing away excess FA ime, it works, but a better option is just separating the fumarate cactus alkaloids from most of the excess FA before washing, by dissolving them in minimal cool distilled water. Excess FA is much less soluble in water, mescaline hcl/fumarate salts are extremely soluble in RT water, FA not so much. Perhaps even try dissolving your fumarates in minimal warm distilled water, and putting it in the fridge. FA should crash out, mescaline fumarate should stay soluble so long as you used enough water.