I am not using the CLK1 input on the AD9510. Can I just leave it floating?

Generally speaking, this is a bad idea. Allowing unused inputs to float, particularly sensitive nodes such as low jitter clock inputs, raises the possibility of external noise coupling into the device. This concern can be resolved by simply applying a .01uF capacitor from CLK1 and CLK1B pins to a clean DC supply or ground plane such as AVDD or AGND. This will provide AC ground locally to the input, ensuring that noise will not be able to couple into the device.