Using cocopeat and feeding cucumber to your millipede is not the way to keep them healthy and happy.
In the wild, they eat humus, rotten leaves, and rotten wood. You can collect from the mountain.
They eat rotten leaves that have fallen to the ground for at least 1 year. So if you see the leaves from the jungle floor, open it up, collect only those from the bottom, the ones soft and can easily be crushed by your hand.

After collecting the leaves, you will find the substrates under the rotten leaves, it is black; that is humus, they eat that too.
You may also feed your millipedes cucumber, mushroom and fruit as snacks.