Bugs on my potted tomato plant
These are aphids (the green insects; the white things are their shed skins). They feed on plant sap, and while their populations can explode fairly quickly, they are feed upon by numerous beneficial insects. Because of this, control with insecticides usually isn't necessary. You can blast most of them off with a strong jet of water from a hose (if one is available, as we note you have them on a balcony), and they also squish under fingers easily. If their numbers keep increasing and significant plant damage is occurring, you can try insecticidal soap, though do not spray when temperatures are above 85 degrees F to avoid plant damage. For best effectiveness, thorough coverage of all plant surfaces is important (you can skip over any open flowers), and a few re-applications may be necessary. Check the product label regarding intervals between sprays and when to cease spraying before harvest, if applicable.