toilet gadget

I always wonder, and also almost forget to ask. But not today. I
remembered, so here is the question:

For years I have been wondering about a small yellow disc, mounted in
many (not sure whether Only Boeing or whether Airbus also has them)
planes inside the toilets, usually on the wall, above head level.

The disc is round (duh!), ca 5cm in diameter, ca 1cm thick, and
consistes of a base, with a rim and inside there are about 16-20 spikes,
the same length as the rim is thick, so they don't protrude. The whole
thing is usually mounter in some kind of half-circle holder, and of a
transparent yellow colour. If the rim wasn't there, it would look like
some kind of hard plastic brush.

What is that thing there for? It must have some purpose, but it totally
eludes me.
(And yes, I forgot to take a picture (again). A Google search for terms
like 'airplane yellow disc round plastic toilet' did not come up with an
explanation, nor with a picture).

Any insights are appreciated
M