Sorry for no early response. First, let me say that I am not
knowledgeable with insect identification, but I suspect what you have on the
canna leaf is a family of aphids (both mature and young). They need not be hard
to control and suggest you do a search for aphid control. Aphids in their own
right will not do serious damage to mature cannas BUT they a considered a
vector for the spread of disease and this is a major concern for their
infestation on most plants.

With the above stated; let me say that depending upon your
location, cannas typically begin to complete much of their growing cycle and
can show deterioration of the mature stalks.

Kent