[SOLVED] Need help with my Mermaid Tail!

hi. First of all, your tail has no proper uv_1. Without it, the tail will not deform during body customisation. I suggest you to separate accessories you created, delete the original tail, append it again(it will have the original uv_1 map that was deleted), transfer the uv_1 map from it to your accessories, combine them.
As for your issue, it is caused by texture overlapping. One of the items your sim wears on these pics has its uvs in the same place that your tail accessories.

You did not transfer the uv_1 map for accessories. Just checked the ea uv_1 map for this (ea) tail and I HAVE TO LAUGH, they half-assed it, these huge uv island should not be there obviously.
1). Select your tail, choose uv_0, switch to Edit mode, enable sync;
2). Press B, select uvs for accessories;
3). Disable Limit selection to visible;
4). Holding Shift-Ctrl, lasso-select as much of these,um, fins you added as you can without touching the tail itself;
5). Select - Linked to select the rest of these fins;
6).P - Selection to separate;
7). Append the original tail with ea uv_1, select fins+accessories mesh, in Modifiers tab add Data transfer. Set it like in the pic choosing appended mesh as a Source object;
8). Apply;
9). Delete your original mesh without uv_1 map, with fins+accessories mesh selected, Shift-click on appended tail and combine with Ctrl J;
10). If you want you can scale down there huge uv islands in the back, move them closer to where they should be, select each vertex in 3d view, press W, choose weld, move the vertex where it used to be

Just wanted to give an update: i made it!! The problem was the location of the accessory texture! I had to scale the little fin part down, to make place to put the texture there. I tried this technique in the very beginning, but it didnt work because ive putten the texture on top of the actual tail part and scaled everything down together.
Thanks for the help anyways