White spots on mint leaves

What are these things and what can I do against them?

Looks like either spidermites or white flies, both are problematic for mints and other herbs.
Check the undersides of the leaves to find the critters.
You can use a gentle soap on a rag to gently wipe off the leaves and clean off the pests.

Bugs bites of some kind or other... I love a jet of water from a hose for dislodging bugs. Many of them will drown, or get lost.

Looks like thrip or other leaf miner damage to me. I'd like to echo the mild soapy water and water jet that Redhawk and Jospeh mentioned. Ladybugs (their larvae including) and green lacewings also eat leaf miners, and spider mites & whiteflies.

I agree it is some type of bug.

Look behind the leaf for them, might need a magnifying glass depending on how good your eyes are.

Hitting the leafs with a hose, top and bottom will knock down a lot of the bugs.
If important to you, identify the bug and see about getting predatory bugs.
Depending on the pest you might need to treat the soil and the leafs such as if they are thrips.

Horticulture soaps are good too but not if you are going to use predatory bugs.
In future, you might consider planting some companion plants that attract good buds and/or that are more attracted to that plant and your plants.

Best of luck!

Thank you! Now I discovered tiny white bugs flying around the plants. I will try soapy water.

I have the same problem. If the plant is otherwise healthy, i disrupt the life cycle of the critters by cutting the plant way back. I regrows vigorously and the new leafs are softer, tastier and bug free. Can be used in salads, sauces and such.

The stuff i cut of, i stil use for tea. I remove damaged leaves and then dry it. The rest (damaged leaves) goes into a odourmix (my wife likes to use them in her kindergarten class to scent the classroom) or is used in the drum with nettles etc... to make liquid feed.

I prevent most damage by spreading the mint plants over the garden and by putting them in pots with other stuff. The pest only really bothers me where i have lots of mints near each other.