If you have new growth on a succulent plant, aphids are interested. And I'm seeing aphids (and other insects) everywhere-- roses, oleander, spirea, river birch. It's unending.
So why now? Aphids are fairly delicate insects with thin exoskeletons (compared to say, a beetle) so when we get into brutal summer temperatures, their numbers dwindle. Water is more scarce, plants have hardened off...I call it "summer dormancy" and it seems that all of our temperate plants (and people) slow down and just focus on survival when the Fahrenheit stays in the 90's and above.
Aphids feed on plant sap, the high sugar liquid found in the phloem (and elsewhere) of plants. Phloem (for those that haven't been in botany class recently) transport carbohydrates produced by the leaves down to the roots, flowers and fruit.
How do you keep from having aphids? For one, don't overfertilize with nitrogen. Plants that are pushed with synthetic fertilizers often have higher aphid populations (it's like junk food for aphids and they are addicted). If you promote slower, more sustained growth, you often have less insect pests.
Second, diversity your garden and work to attract birds and beneficial insects into your garden. A garden with habitat for birds, native plant species incorporated into the landscape and high plant diversity will avoid outbreaks and epidemics.
And remember that I said that aphids have thin skins? That means that without a steady diet of sugary liquid, they dry up pretty quickly. If you just have a plant or two with aphids, spray them with a steady stream of water and knock them off your plant. They'll die before they ever have the change to climb back onto the host plant. It's an epic hike back to the tips of an oleander branch if you're a tiny aphid.