The 6500k bulb would be much better for plant growth. This is because plants really only use light from the 5000k-10000k range and the 3100k isn't even in that range. 6500k is known as the sweet spot in terms of plant health and growth, so that would be your best option.
Now, the other problem is the wattage of your lighting. It is recommended to have at least 1wpg for a planted tank, and you have about 0.60wpg. Now, i dont know if the marimo moss balls need much light, but i know the hydrocotyle japan (a.k.a pennywort) needs low to medium lighting, which you have very low light so i think it will probably slowly die off or just barely hang on without much growth at all. The best option for you to do is to get at least 1wpg by either buying another one of the fixtures you have, getting a cheap dual t8 shoplight at lowes, or getting a cheap dual t5 aquarium lighting fixture like the odyssea dual t5 for $45.