OK, it's good to know that I'm not the only one with this problem, because I really couldn't find much on it. The only solution I found was to change my mermaid back into a regular Sim by eating 2 Mermadic Kelps, then another to change him back into a mermaid, but that did nothing. One of his daughters is also an alien/mermaid hybrid, but she can have a tail. However, her skin is always blue when she's not a mermaid. I can't seem to give her a "normal" disguise. But I'm OK with that. Thank you for your input! Just another case of putting up with EA glitches, I suppose, lol.