Can I leave XTAL2 floating when using an oscillator chip with 1 output and no input?
To quote the information from the datasheet of the part you referenced:

However, a 6-MHz oscillator may be used by connecting the output to
the XTAL1 pin and leaving the XTAL2 pin open.

So yes, you should leave XTAL2 open if driving XTAL1 with an externally generated clock signal.