hmm... sorta like the resin you see seeping out of old pine boards?
If so, it sounds like some of the pieces probably weren't old/weathered enough.
I'd still try boiling, maybe with a piece of sacrificial wood/cloth/something absorbant to try and remove the existing resin from the pot, and if that works, try repeating with the mopani alone. You might also be able to just physically remove most of it by picking at it or something (I'd still give it a boil afterwards). This might work better if you can completely submerge the piece you are trying to clean, (throw in a brick, a rock, or a bag of gravel to hold it down if need be), and also put something absorbent up top to attract/gather the resin as it boils off.
Just to say, I haven't done/tried this myself, so I have no idea whether it will work, or how well it will work, I'm just trying to toss some ideas out there, it would be a shame to see your mopani wood go to waste.