Houseplant Ivy

Question

My three year old Ivy, although looking healthy and robust, has started giving off a sticky substance. It grows in a light filled windowsill and is watered once a week with feed once a month.

Answer 1

This is called honeydew and it's the residue from pests. Spider Mites are a common pest on Ivy, though it could be others.
Treat the Ivy with Neem Oil.
Make sure you keep this plant away from others until the pests are gone.

https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
https://www.gardeningknowhow.com/garden-how-to/info/what-is-honeydew.htm