This is most likely calcium carbonate (calcite) scale. With acrylic, you don't want to use anything abrasive. There is probably scale all through the equipment and plumbing as well. If your fill water is lower in calcium, then you should refill to get the calcium level down.
Then, turn the heat way down, because calcium carbonate dissolves better in cold water. However, not so cold as to risk freezing, if that's a possibility. Next, maintain the pH at 7.2 for a while, and the calcium should begin to dissolve. By maintaining the pH at 7.2 (but, not lower), the TA (Total Alkalinity) should begin to drop, which will help the scale begin to dissolve.
Ideally, you want the TA and calcium as low as possible. Distilled or deionized water would be great if it were available. You can drop the TA by lowering the pH to 7.2 and then aerating by running the blower and jets. Don't allow the pH to drop below 7.2. Keep the heater off until the scale is dissolved. Hot water reduces the ability of the water to dissolve scale, and you could burn out your heater element because it can overheat when it has a thick coat of scale.
After a few days, you can drain the tub and refill. If there is still some scale, you can use some distilled white vinegar, or a product such as
Descale-It Pool & Spa Cleaner* (Contains Hydroxyacetic Acid (Glycolic Acid))(or other descaler formulated to be safe to use on acrylic) to remove the remaining scale.
Maintain the CSI at less than 0.0 from then on.
*Note: Always check with the tub manufacturer before using any chemical product on your tub to ensure that it is safe to use. Also, try a test spot to make sure that it does not damage or discolor the material.
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