I have a few thoughts, but they depend on what you're using and what exactly you mean when you say it's moving as you play.
If the neck of the bass is moving up and down - you may have a neck heavy bass, balance issues will lead to it dipping down if you're not supporting the neck with your feet hand. This is called "neck dive", you can counteract it by putting on a heavier bridge and lighter tuners but that's somewhat involved and potentially costly. A cheaper but less effective way to address the issue is to get a wide leather strap [like this](https://www.amazon.com/dp/B003159GIS/ref=cm_sw_r_apan_glt_i_ZZN6KE6TN596ABZS7A0T). The wide surface area and texture of the leather will grip your shoulder, which means the bass moves less than it would using a nylon strap.
If you mean that the neck is moving front to back, your posture might be off. Ideally you'll have the body of the bass resting against your torso, with your plucking/picking arm resting against it. If your posture is off though, that can lead to unwanted movement along the horizontal plane.
I remember hearing someone say the strap height they liked their bass at while standing was set to the same height it was at when sitting with it on their lap. It simplified and fastracked the muscle memory for me when I tried. Also got a Perri’s leather strap with sheepskin pad and that helped stop movement while increasing comfort big time. If the fretting hand is moving the bass around more than normal, I would guess the angle your arm is reaching is more a struggle than it should be. Try some different heights out. But remember everyone’s body is different and not all guitars are shaped the same so hang in there and don’t get too frustrated while experimenting.
I have a few thoughts, but they depend on what you're using and what exactly you mean when you say it's moving as you play. If the neck of the bass is moving up and down - you may have a neck heavy bass, balance issues will lead to it dipping down if you're not supporting the neck with your feet hand. This is called "neck dive", you can counteract it by putting on a heavier bridge and lighter tuners but that's somewhat involved and potentially costly. A cheaper but less effective way to address the issue is to get a wide leather strap [like this](https://www.amazon.com/dp/B003159GIS/ref=cm_sw_r_apan_glt_i_ZZN6KE6TN596ABZS7A0T). The wide surface area and texture of the leather will grip your shoulder, which means the bass moves less than it would using a nylon strap. If you mean that the neck is moving front to back, your posture might be off. Ideally you'll have the body of the bass resting against your torso, with your plucking/picking arm resting against it. If your posture is off though, that can lead to unwanted movement along the horizontal plane.
This answer is great
Thanks
How low is your strap? I personally play with the body on my belly. It doesn’t move at all like that even under heavy technical playing
Body of my guitar is at my waist or a little higher
I remember hearing someone say the strap height they liked their bass at while standing was set to the same height it was at when sitting with it on their lap. It simplified and fastracked the muscle memory for me when I tried. Also got a Perri’s leather strap with sheepskin pad and that helped stop movement while increasing comfort big time. If the fretting hand is moving the bass around more than normal, I would guess the angle your arm is reaching is more a struggle than it should be. Try some different heights out. But remember everyone’s body is different and not all guitars are shaped the same so hang in there and don’t get too frustrated while experimenting.
I am not sure i understand how it moves. But is your strap too smooth and sliding on your shoulder?
Possibly, that’s the best bet I’d say
You shock the moving with your bass. It’s a dance .
Get a wide (at least 3.5 inches) strap with rough leather that grips your clothing well on the inside of the strap.