It's hard to tell exactly from the pic, but it looks kinda like leaf miner. Leaf miner is a tiny insect which burrows between the top and bottom epidermis of the leaf. It wanders around in a random path. From that description you can tell if you have leaf miners and look up the appropriate treatment if you do. Small seedlings get sheared off at ground level by cutworms. The solution is easy. Cutworms must encircle the stem to do their work, so you can block them by inserting a toothpick or matchstick into the soil parallel to and against the stem. Gardening has many opportunities for failure. I hope you have lots of big successes to compensate for the failures. Jim

Hi and thanks for the responses. The cutting is in a different batch of soil than the original, however I have maybe a half dozen other plants in this soil or a mix containing this soil and they haven't shown signs of any diseases. I'll try rinsing it, it is a very small cutting. It's growing a lot slower than mint has grown for me in the past :/ I live in the city, not very close to a nursery but maybe someone at the farmer's market will be able to help me out! Thanks a lot :)

I'm not sure. It kinda looks like leaf miners and it kinda looks like red spider mite damage (check the underside of the leaf for tiny webs or the mites).

I do not recognize this so I Googled it. This might be helpful https://www.google.com/search?q=black+spots+citrus+leaves&biw=1680&bih=857&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjYltv_39nQAhXCRSYKHXBhDDIQ_AUIBigB#imgrc=3GynaX0KoECnTM%3A