An FL-D filter and an ND filter do different jobs.

An FL-D filter is a color-correction filter. It’s intended to reduce or counter the color cast from fluorescent lighting so the scene looks closer to daylight-balanced color. Because it corrects color, it usually has a visible tint rather than looking neutral.

An ND (neutral density) filter is used to reduce the amount of light reaching the sensor or film without ideally changing color. It’s essentially a gray filter. Photographers use ND filters when they want slower shutter speeds or wider apertures in bright conditions.

In short:

• FL-D = changes color balance to compensate for fluorescent light
• ND = reduces light intensity while keeping color as neutral as possible

For digital photography, FL-D filters are less common than they were in the film era because white balance can often be adjusted in-camera or in post, especially when shooting RAW. Also, fluorescent lights vary a lot, so a generic FL-D filter may not perfectly match the specific lighting you’re under.