Sticky white residue

Hi all
Newbie here.
I've been cultivating palms and cycads for a few years. Painfully slow growing in SW England and not much reward for my efforts. Having said that they are beautiful and add a touch of the exotic to my garden. I only put them outside for the summer months and take great care over them.

However I am finding an immensely sticky white residue over the leaves when over-wintering them. I am able to brush it off with a soft-ish scrubbing brush and warm water, mainly because a cloth just won't remove it. The residue isn't particularly fussy about which plants it afflicts, and even appeared over the leaves of a chili plant I'd successfully managed to over-winter!

It doesn't appear to harm the plants in any way. Does anybody know what it is and how to prevent it this winter?

Martin.