Milky white resin happens for 1 of 2 reasons. Moisture and Temperature.
If you mix your resin below the optimal temperature (70-75 degrees Fahrenheit) you risk this happening. If you're pouring and your resin is coming out in thick "clumps" it is likely too cold. When you catalyze the resin and hardener together microbubbles will get trapped while your mixing and start to multiply creating a bubbling, milky mess. If you've already poured this onto your piece of work, you may be able to remove the bubbles by torching it. There is still a chance though that those tiny microbubbles will be stuck below the surface because the resin was not at optimal working temperature.
Another reason why your resin has become milky is that moisture has come in contact with the resin. Be sure that your mixing utensils and bowls are clean and dry before starting. The smallest amount of water can effect resin in a negative way.
If you take the time to make sure your resin and working area is at an optimal temperature, this should prevent any problems with milky resin.