The small triangle on a water meter is not supposed to be moving slowly or at all with no water being used.
If the small triangle on a water meter or the small dial on the water meter is moving and no water is running it means that you have a small water leak somewhere past the water meter on your side.
When the water is not running the small triangle on a water meter should be completely still, although sometimes you might notice that it turns a bit when you turn the water off, but it should not continue moving with the water turned off.
The most common culprit of the small triangle on a water meter moving with the water off is a slowly leaking toilet tank flapper that allows water to slowly drain out of the toilet tank and then the water comes on in the toilet to fill back the water tank.
It could also be a small leak in a faucet or underground leak, depending on how fast it's moving.
You can test your toilet for water leaks from the tank into the bowl by putting some drops of food coloring in the toilet tank and then wait for around 20 minutes to 30 minutes and see if you notice the food coloring in the bowl.
If there is some food coloring in the bowl then it means that your toilet tank flapper is not sealing properly and allowing water to flow out and into the tank.
Leaking toilet tank flappers are a huge cause of high water bills due to water slowly or sometimes even flowing fast through the tank.
You can also shut off the water valve to your toilet and check the small triangle on a water meter again and if it stops, then the toilet tank flapper leaking is your issue.
Or sometimes the toilet tank fill valve may not be shutting off fully and continuing to run as well and it may need adjusted or even replaced.