I think that's a wax buildup in the pores of the wenge (which are huge). You can try wiping the top with Murphy's Oil Soap (any hardware store should have it), diluted with water per their instructions. It's pretty gentle, so you don't have to worry about this too much.
If that doesn't work, then you'll need to use a solvent to dissolve the wax, which has a higher risk of damaging the finish. I think woodworkers would use toluene or methyl ethyl ketone, both available as solvents in the paint section of your hardware store. Both of these solvents are strong stuff and flammable, so you need to use care when using them. They should be safe for cleaning use on polyurethane or polyester varnish, but you should ask someone knowledgeable about your bass' finish before using these. I think if you had a varnish finish the pores from the wenge would have been filled and you wouldn't have this problem, so it's possible that you have a penetrating oil finish here.
David Fung
