Changing the density settings for your render should help remove the moire look. In the “Main Controls” rollout of the Krakatoa GUI, you should try playing with the “Final Pass Density” and “Density Exponent” spinners. These values control the apparent transparency of each particle. Enabling the controls on the left of these allows you to make the particles more or less transparent to the light only, so they will render with the same opacity to the camera. Making them less dense (ie. more transparent) to the light should help smooth things out.
Increasing the resolution of the shadow map for your light will have a similar effect.