Yep, aphids. Spray with insecticidal soap.

For such delicate plants you can brush them off with a toothbrush and then spray just the soil.

Why would one spray the soil?

I typically suggest that aphids be squished with fingers and sprayed with plain water.

Since they're lettuces I'd just squirt them off with plain water in a spray bottle. Squishing would work too if you can manage it without bruising the lettuce. I wouldn't use any insecticides. Being on a balcony they might not be exposed to wind or rain which generally keep these things at bay outdoors in the garden.

Toothbrush is superior in quickly getting aphids out of delicate new growth that water bottle spray might not. Use a toothpick for tight places.