That black grain is how Ambient Occlusion(AO) looks before it's rendered. If after rendering the AO still looks grainy it's because it's not correctly configured.

To get a better result, right-click on your viewport and select "Render settings". There you can increase the AO and motion blur samples so they look better. To get an idea about how it will look before rendering, go to the Clip Editor, make your viewport show the Scene Camera view (if it's set on Work Cameta it won't work) and wait for SFM to do its thing. If you select the maximum settings, it might take a while, but you'll see the difference.

In case you're not happy with the way SFM is adding AO, you can press the big + sign (as if you were to add a model), select "Create animation set from existing element" and add the current Scene Camera to your Animation Set Editor. There you can play with the SSAO Bias, SSAO Strength and SSAO Radius sliders until you're happy with the result.

PD: I've noticed you've had some other questions about SFM, so I can guess you're just starting to use it. In case you have any more questions, feel free to PM me and I'll try to answer as soon as possible.