Context:
In a dungeon, a player was stuck on a pillar, and from below a spider was climbing upward. The player cast Grease underneath the spider to try to make it fall back down.
While neither of the spells have anything specific in their descriptions that would suggest that one renders the other useless, a quick train of thoughts made this question come to mind.
As is stated in Spider Climb:
The subject can climb and travel on vertical surfaces or even traverse ceilings as well as a spider does. The affected creature must have its hands free to climb in this manner. The subject gains a climb speed of 20 feet; furthermore, it need not make Climb checks to traverse a vertical or horizontal surface (even upside down).
This means the player is able to climb any surface without a climb check (Which should include slippery surfaces, like the one created by Grease).
However, in order to traverse the slippery surface created by Grease, Balance checks are required, as is stated in Grease's description.
Now, how should these two interact? I had a few options in mind…
- The required Balance checks should be turned into Climb checks when Grease is cast on a vertical surface. People with Spider Walk don't have to roll the Climb check.
- Spider Walk completely negates Grease, rendering it useless.
- Grease affects subjects of Spider Walk no other than creatures without it.
EDIT, 8/2/2015
I found this.
Icewalking (Ex): This ability works like the spider climb spell, but the surfaces the dragon climbs must be icy. It is always in effect.
Freezing Fog (Sp): An old or older white dragon can use this ability three times per day. It is similar to a solid fog spell but also causes a rime of slippery ice to form on any surface the fog touches, creating the effect of a grease spell. The dragon is immune to the grease effect because of its icewalking ability.
Emphasis mine. This seems to imply that either Spiderclimb negates Grease, or this is an exceptional scenario, implying it normally wouldn't.
Best Answer
The answer is in your bold letters.
The description says that it doesn't need to make Climb checks in order to traverse a vertical or horizontal surface. In that case, the check is to overcome the slippery effect of Grease spell. I would assume that it is the same as saying that a creature with walking speed needn't make any Balance checks to walk on an horizontal surface. But it still needs to make them in order to overcome obstacles. Grease is an obstacle, not a surface.
The spell calls for a Balance check, but given the specificity of the situation you describe, your GM could perfectly call for a Climb check instead. It seems reasonable.