Yep - that is indirect illumination bouncing off the diffuse underside of your donut.
You may be able to reduce it by setting your diffuse bounces to 0 or 1, but anything higher and it will remain. It’s just a consequence of the path tracing engine that cycles uses, coupled with the low light environment of the shadowed area and the low number of samples you are rending with.
Cycles renders always come with a tradeoff - noise vs speed. More speed = more noise - that’s just the way it is.
You could always try downloading one of the experimental builds with denoising function built in. That may also help - but does have a slight performance overhead and it can introduce other image artifacts. Again - it’s a trade off between speed and accuracy.