Why is 10W resistor getting hot with only 6.5W running through it?
First let's do a quick number crunch: 

6.528W/10W = 65% (of 10W)

Referring to the datasheet:



There is about a 165C rise in temp. Do not touch!.

As for "Is it a safe temperature for the resistor?", refer to the next figure:



I'll admit that the Derating Curve Graph kinda hurts my head. But, if you follow the 10W curve over to 25C (about room temperautre), the resistor should be able to handle 100% of it's rated power. Note that I'm only assuming the ambient temperature is 25C! If you have it lying on a brick, it should be okay.
It appears  that the resistor can handle up to about 115C ambient temp @ 65% load. But that would be pushing it to the max.