No reason to announce slowing or stopping, if you're riding properly on your own you're watching out for obstacles and people alike and leaving enough space for your to pick your line out almost regardless of what happens in front of you.
If you've got people treating it as a road group ride and drafting or something then it's appropriate, but it's much more of an at-your-own pace kind of thing... saying slowing or stopping would mean that you expect the people behind you to do the same.
Regardless, if you're following leave enough space to do what you need to do despite what he does, if you think that's too slow then maybe you shouldn't be following.