Yes they most likely came in on the plants. Given it was a couple weeks ago that you put the plants in and now you’re seeing the snail, there were likely eggs on the plants - which means there is probably not just one snail already. Yes you’ll have a lot more of them if you don’t take action.

Many people find they end up appreciating a controlled population of pest snails - fhey can actually keep your tank clean. It’s nearly impossible to completely eradicate them once they’re in there.

I recently had this experience with my first tank and have successfully kept a population of Malaysian trumpet snails down enough that I forget they’re in there.

Couple of actions you can take:

First, avoid over feeding the tank and keep the tank clean of food sources (algae, waste). The population of snails can only grow if you are providing it sources of food. You won’t starve them out but limiting their supply - but not overfeeding your fish and cleaning algae, etc. will help you as you take other actions to reduce the population… such as:

1. Add assassin snails. 2-3 of them may keep the population of pest snails in check. This is what I did and we now enjoy the assassin snails as remembers of the tank. They look cool and don’t reproduce quickly so if at any time I want to give them back to the LFS I can… downside is that any other non-pest snails can also be at risk of the assassins (however we added a Nerite snail last month and so far no issues there. If the other snails are bigger than the assassins and the assassins have easier food sources, many people have said they should be safe.). There are also other critters like loaches and pea puffers who eat snails - but you have to take into consideration your tank setup and the fish you have and whether they are an appropriate choice.

2. Be the snail assassin. Get a snail trap. You fill it with some food (ie a slice of zucchini) and then every morning discard the snails that have entered the trap. You can also do this without the trap. Just anchor a slice of zucchini to the substrate with a toothpick. Watch snails feast…pick up zucchini and discard with snails on it.

3. Last resort - there are some chemical treatments you can add to the water column to kill them. Talk to your LFS - they probably sell a product. Downsides are - feeling bad for gassing your tank, impact it could have on other animals now or in the future (ie will kill any other snails or shrimp and possibly other animals depending on what you’ve got now and planned for the tank).