Sounds like mealy aphid infestation - other aphids which infest plums usually cause curled, distorted leaves. You can spray with Bayer Provado Ultimate Bugkiller now, but don't harvest plums for a minimum of 14 days after spraying. Alternatively, Westland Plant Rescue Fruit and Veg Bugkiller, when you should wait 7 days before harvesting fruit. You will also need to carry out a winter wash treatment - these aphids overwinter on the tree. Vitax Winter Tree Wash is one, and should be applied on a mild, dry day during winter, when the tree is dormant. You will also need to keep an eye on the tree from next spring and spray with one or other of the first two products mentioned at any sign of infestation, though usually obvious infestation doesn't occur till summer with mealy aphid. Don't, though, spray when the blossom is present. It is necessary to treat the whole tree, and that includes the winter wash, so hopefully, your tree isn't too large to be able to do this.

I have had the same problem, will try the winter wash. hope it works for you tracyb999