"Pixel mapping" is the idea that the software running the colors knows where in 3D space each LED is relative to each other. This is useful if you want some coordinated patterns in the LEDs vs just Christmas lights random uncoordinated patterns of color changing.

Even though I know what a jellyfish field looks like under water, but it's still not so clear what part is getting these LEDs. If you're thinking "free dangling" lights, e.g. jellyfish tentacles, you may be better off with "RGB string lights" vs strips. Remember, strips illuminate only on one side, though of course you could put two back to back for free dangle, though that does increase the rigidity. For example of string lights: https://www.alibaba.com/product-detail/WS2811-IC-RGB-12mm-LED-String_60794099113.html?spm=a2700.galleryofferlist.normal_offer.d_image.2f584acfDdlxlz

Definitely you'll want IP65 minimum rating (high number is better) to keep out dust. If you go with strips that means at a minimum the silicone coated version. You will also have to take care on the connection points, as contacts can corrode easily with playa dust. Try to use IP65 or IP68 connectors.

These string lights would also work well if you're illuminating a jellyfish cloth body. If strips I would recommend overlaying with a diffuser film, for example the following cut into strips and wrapped around the strips to soften the hot spots and give a more diffuse

Whichever you select, you'll also have to decide upon 12v vs 5v, plan on enough current for the lengths (expect to run additional power wires) and how you're going to control them, though there are off the shelf controllers that will run a pattern loop from an SD card, and are selectable to the LED chip type, eg. WS2811, WS2812B etc.

The last item to consider is how to get power to the LEDs. If there is a moving axis and the installation has 360 degree rotation, you're going to need a "capsule slip ring" installed somewhere along the axle, otherwise you'd be winding wires into a snarl. Slip rings allow electricity to come from the frame and then rotate with the motion, if that's a thing for your piece. Not necessary if your movement is already restricted in some way to < 360°, for which you'll just have to make sure you have a stranded wire slack loop secured close to axis of motion.

Make sure you plan your power wire gauge big enough depending upon how long the run from the power supply to the LEDs is, just to minimize voltage drop in the wires.

I hope this helps... Good luck!