you need to look for aphids or any other sap sucker. it sounds like honeydew which is the waste left from aphids etc. if you have any then sort that problem and then wash the leaves down with a fine mist from a hose if it is a big plant.

Have a look at the underside of the leaves, and if it is an aphid infestation then spray the leaves with a soapy water solution, I use washing-up liquid. One or two applications usually does the trick and unlike most commercial products it's cheap, it's readily available and it won't do any harm to the environment except the aphids of course.