Is this safe?

Mold is just part of a cycle that will correct it self over time. I don't have much knowledge about slugs in with frogs, but I do know slugs can do alot of damage to plants. If your looking to have micro organizims in your substrate, springtails would be a good start.

Well, the slugs haven't done damage to any of my plants yet. What is the attack method of the slug? Roots and/or leafs?

The will usually damage leaves and areas of new growth.

I have two slugs in my viv. They have lived a lot longer than I thought they would. They seem to help a little with just a little damage to plants, a couple of holes in some of my plants with bigger leaves. Earth worms are great to. Good for your soil! I would not recommend getting them from out side. They are so cheap just buy some. I also have added some super worm beetles. They love to eat the poop! Helps keep things clean. But the spring worms would help a lot too.

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