Hi, Please this is an at distance guess and you should seek the advice of a local professional.
Sap sucking insects isert a probiscus into the plant and remove sap.
As I understand it the sap does not contain a good balane of desired nutrients and in particular nitrogen required for growth. A black mould then grows on this sap. You can create the same effect by spraying maple syrup solution to give rockwork an aged appearance. If this is the case then the insect problem can be managed in a number of ways.