i would like to know first if they are R.Yuma or R.Florida.
whichever they are i would like to know your lighting, how high or low these were placed in your tank, and also how long have you had them.
sometmes yumas get to much light and melt away. as do Florida ricordias.
but if they are newl purchased and recenlly added to your tank, yumas also tend to simply "melt" away when newly aded.
just happend to me the other day, i lost a solid hot pink yuma polyp, it just vanished, while all my others are doing fine.