Are you sure it is sand and not detritus? Pinch some in your hands and describe the texture.
Particulate organic matter that floats around in the display tank can settle out of the water column when there is not a lot of flow. This POM settles and forms what we call detritus.
What other equipment do you have? is it just the pump or are there other filters involved that are running into your sump. Sounds like it was sand that settled after the water was cloudy and it gets everywhere in your system, no big deal, you can siphon it out or leave it in