I have a mature peace lily plant that is developing a sticky sap on the leaves.. What could be the cause and cure?

Question

The sap is dripping onto the surrounding floor.

Answer 1

The dripping is likely to be "honeydew" excretions from a sucking insect pest, probably aphids.

Inspect the leaves and especially the under sides. You can wipe the pests off with a rag or wet sponge to minimize the infestation and nuisance.

You can use neem oil to treat the plant.

https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm