That is the fruit's response to being damaged, usually at this time of year by wasps.

Owdboggy.

CLEAR BLOBS OF HARDENED RESIN / JELLY ON PLUMS
Plum trees which suffer from environmental stress such as hot or cold weather, fluctuating levels of moisture at the roots or similar conditions often produce plums which have small blobs of clear and hardened liquid on them

There can be lots of them or just a few. As well as environmental conditions this can be caused by very localised insect damage or it can be the result of some other more serious pest or disease. The blobs themselves are not a pest or a disease but they are produced as a result of some other pest, disease or condition.

Cut an affected plum in half and look for signs of a pest, if none is found then the plum is still edible. If there are other signs then diagnose what has caused them rather than worry about blobs themselves, they are just a sign of another problem.