Hi and
Some insect pests such as spider mites form a web to protect themselves, but the web usually starts where they are feeding, such as undersides of leaves and the growing tips of the plants. Is there any webbing on the plants in your greenhouse?
In the soil, a common houseplant pest is fungus gnats, which are small flies that crawl around on the soil.
Regardless of what pest you have, I would recommend spraying with houseplant insecticide inside the greenhouse, covering both plants and soil, at least once a week until the insects have gone.
