Springtails
The small white bugs in your terrarium are most likely springtails. Springtails are not dangerous to you, the plants or other animals in your terrarium. In fact, they are very beneficial and provide a bioactive substrate by eating waste products and thereby providing natural fertilizer.
Technically speaking, springtails are not even insects. If you want to learn more about them, be sure to check out the article about springtails. There you'll find a lot more indepth information about the little residents living inside your terrarium.
If you're looking for springtails, you can also easily buy them on Amazon.
Woodlice (or isopods)
Another possibility is that the small white bugs are woodlice. Not all woodlice are gray or brown in color, as are most found at home. Some of them are white or even colorless, as they lack any kind of pigment. Many animals that spend their lives in darkness have no pigments, as they would not be seen without light anyway. You don't have to worry about your terrarium with woodlice either. Just like springtails, they feed detritivorously (i.e. on plant debris) and thus clean up your terrarium.
Here you can find a detailed article about woodlice. It's amazing how many species there are. One species even looks a little like a rubber duck.
Glasshouse whiteflies
Another white insect that likes to stay around plants and especially in closed terrariums is the white fly, also called glasshouse whitefly. It especially likes to eat soft leaves and also lays its eggs underneath. If you don't act quickly enough, they can wreak havoc on your terrarium.
In this article here you will find everything about whitefly and what you can do against it.
Neither?
If the little white bugs are not springtails, woodlice or whiteflies, watch them for a while. As long as they don't over-eat your plants or reproduce non-stop, you can sit back and let the little ones be.
If you need to do something about them, I strongly advise against pesticides or household products like vinegar. These affect the pH value of your substrate immensely and can also harm your plants. A sensible first step would be to put a small spider into the terrarium. It will be happy to get plenty of food and will not harm your plants.
By the way, here's another in-depth article on 6 types of insects and other animals for your closed terrarium.
