How much movement is "creepiing"?
A small (fractions of inches) is comparatively normal, particularly if you are on an incline and gravity is in play.
On the other hand, if it is a sustained and considerable, ( more than just barely perceptable), then you may need to have the HST pedal adjusted.
Three HST tractors, a 5030, a 5740, and a B3000. All have had some "barely perceptable" movement with the pedal in a neutral position, and particularly on inclines. If I am under load and in tight quarters I will hold with the brake, or feather the pedal in the opposite direction just enough to maintain "hover".