Hi Ana –
If you are washing off the leaves and the shiny substance comes back it is most likely an insect pest. Is the shiny material sticky too? If so, I suggest looking for the following:
Spider Mites
Aphids
Thrips
Scale Insects
All of these insects are very small and may live on the undersides of leaves. They feed by sucking the juices and scraping at leaves and stems. As they feed, they secrete a sticky fluid known as honeydew. This shiny goo drips onto plants, attracting ants and promoting a black sooty mold growth on leaves.
Please take a closer look. If you still do not see any insects, could it be that the honeydew is dripping from an overhanging tree?
Good luck!