What kind of plant is this seedling? I've been told it looks like a tomato plant but I'm not sure that's what it is. It's been growing for about a month and a half.
Yes, this is a tomato. At that size, you should be able to smell the typical tomato green scent if you lightly(!) touch or rub one of the two upper leaves.
It’s a regular leaf type and if it’s really already over a month old, it’s about time you transfer it to it’s own pot - it’s a bit stunted. I would have guessed an age of two to four weeks after germination, but some are just slower, especially if they have to compete for nutrition and are in a cooler environment. But the light seems to have been good so far.
If it’s a seedling from a random seed, it could be interesting to see what kind of fruit it brings. If you don’t want to keep it, take it out before it overshadows your succulent, as it will grow faster if it can grow at all.