I assume you mean frictionless surface - so no forces are acting on the trolley.
The mass of the trolley is decreasing, yes. But an acceleration requires a force, and no forces act upon it. Therefore there is no change in acceleration.
The velocity remains the same also. Momentum is conserved because the sand that falls out still has a horizontal velocity. If the surface is indeed frictionless, then the sand that falls out would continue moving along the surface underneath the trolley. The sand hitting the surface would stop its Y velocity but not its X velocity.