I just want to agree with others that it's not possible to tell from your pics the change in color in the anemone. The lighting of the pics makes it impossible.
In general nano cubes do not come with enough lighting for a bubble tip anenome, unless you specifically buy a halide fixture that fits a nano (or T-5).
I bought a halide unit for a nano, via Sunpod (vendor). There are a few others, like Biocube and Aquapod both have halide lighting units available for a tank your size. But be sure to check the measurements of the unit so that it will fit the length of your tank.
If you don't want to invest in a better lighting unit, I would give that anenome to someone who has more appropriate lighting (just my personal opinion).
They also need stable water parameters which can be challenging in a 12 gallon tank. Larger tanks have more stability with ph swings, etc.
All that said, your anemone does not look like it's dying to me. But the loss of its zoanthella (color) is something that happens when they are stressed/sick.
