Rfork--I don't know that you would call it a "hatch," but be sure you have some terrestrials, especially 'hoppers. Most (though not all) of Slough Creek flows through meandering meadows, and that translates to ants, jassids, and in mid-summer 'hoppers. You won't go wrong with a 'hopper and small nymph dropper approach, or at least that was true the last time I fished Slough Creek (it has been several years).
You will likely see lots of feeding fish, and delicate approaches and presentation will usually draw action. Cutts aren't the most difficult of trout to draw to a fly, by any means.
If you haven't been, you'll love the area, and don't overlook some of the other streams in Yellowstone which get far less pressure than Slough Creek.