I don't think that little leaf could support the weight of a slug. Unless it was a little one but you can always tell if it was slugs by the slime they leave behind. My guess is it's a caterpillar of some kind, a larvae form of something. Really you should spray the plants and use a product that has Eith of the insect ides listed below...I got this off the net. It's pretty helpful
Bacillus Thuringiensis
Bacillus thuringiensis is a spore-forming bacteria that naturally exists in the environment. It must be consumed by caterpillars for it to be effective -- upon ingestion, B. thuringiensis releases an endotoxin that creates pores in the insect gut, causing the insect to poison itself over the course of a few days. It is important to match the subspecies of B. thuringiensis to the pest in question; caterpillars respond variably to subspecies kustaki and aizawai. B. thuringiensis requires frequent reapplication because it breaks down readily when exposed to sunlight. The risk to waterways, beneficial insects and honeybees is low when using B. thuringiensis. The acute risk to humans and mammals is very low.
Spinosad
Spinosad is composed of spinosyns A and D, byproducts of the aerobic fermentation of the filamentous soil bacteria Saccharopolysora spinosa. It controls caterpillars primarily through ingestion but can also be effective when in direct contact with caterpillars. Spinosad causes the nervous system of insects to continuously activate, exhausting them within two two days. Spinosad comes in both organically acceptable and nonorganically acceptable formulations, so check product labels carefully before purchasing. Caterpillars may develop a resistance to spinosad; it is best used in rotation with other organically acceptable insecticides. Waterways are at low risk of contamination from spinosad, but it creates a low to medium risk for beneficial insects and a medium risk for honeybees. The acute risk to mammals is low.
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Neem Oil
Neem oil is derived from the seeds of Azadiracta indica, the neem tree. This southern Asian native has long been believed to hold pesticidal and medicinal properties, but it wasn't until 1959 that Western civilization took notice. Neem contains up to 70 chemical compounds, but azadirachtin is considered the active ingredient in most formulations. This chemical acts as an insect growth regulator and discourages feeding and egg-laying by a variety of insects. Neem oil is more persistent than other biological insecticides; but it is slower acting, controlling insects by preventing the molting of young larvae and discouraging older generations from laying eggs on treated plants. There is no known risk to waterways from neem oil, only a low risk to beneficial insects but a moderate risk to honeybees. The acute risk to mammals is very low.
Pyrethrins
Pyrethrins are derived from the powdered flowers of Chrysanthemum cinerariaefolium, C. coccineum and C. marshalli, southwest Asian natives. It is a fast-acting compound that causes continual stimulus of the nervous system in treated insects until they die from exhaustion. Take caution when purchasing pyrethrin insecticides; many are mixed with synergistic materials that are not allowed in organic growing systems. Pyrethrin must be reapplied frequently when pests are present, because it breaks down quickly. Waterways are at high risk from pyrethrin runoff, and both beneficial insects and honeybees are at a medium risk from this chemical. The acute risk to mammals is low.
