The caterpillar in the picture looks like a Hibiscus sawfly larva. They are a common pest of hibiscus, hollyhock, mallow, and other members of the Malvaceae family. Hibiscus sawflies are not actually flies, but wasps that resemble caterpillars. A healthy swamp mallow plant can tolerate a small amount of sawfly damage. However, if the infestation is severe, you may need to take steps to control the sawflies.
To control them you can try the following options:
Handpick and remove the caterpillars and be sure to wear gloves when handling the caterpillars, as they can irritate your skin. Encourage beneficial insects such as Ladybugs, lacewings, and parasitic wasps all of which prey on hibiscus sawflies. You can attract these beneficial insects to your garden by planting flowering plants that they are attracted to, such as dill, fennel, and yarrow. Use insecticidal soap or neem oil which are organic insecticides. Be sure to follow the instructions on the label carefully. Use Bacillus thuringiensis (Bt) which is a bacterium that is toxic to caterpillars. It is an effective way to control hibiscus sawflies, but it will also kill other caterpillars, including butterflies. Be sure to choose one that is labeled for use on hibiscus or mallows and follow the instructions on the label carefully.
Remove any debris from around your plants in the fall, this will help to kill overwintering sawflies.
Water your plants regularly since stressed plants are more susceptible to sawfly infestations.