Along with OFL, I have a hard time seeing the"sandy" substance. If you're talking about the few grains of dust or semi-dissolved solids on the brown prominent rick. No tank stays pristine. All kinds of things build up. If it's not affecting your fish or messing up your water quality, just clean/siphon it up as best you can.
I still think you don't
want shells in your tank. Most keepers in N. America have harder water than is ideal for Betta. Most questions we get are about how to soften water and lower pH.
Also (and I'm sorry, I know you didn't ask for comment), you may find that size rock substrate hard to keep clean. Lots of places for waste to go and decompose. Hard to clean it out of there.