Those are centipedes and they are harmless to the plant but aesthetically undesirable. Did you repot your Marginata? If so, the soil you used may be the source of the centipedes and you may not want to use it again.
There are pesticides that you can apply to the soil, but I don't recommend them. The next time your plant needs water, set the pot in a tub of water filled halfway to the top of the pot and leave it for a few hours. The completely saturated soil will cause the critters to move to the top for air and you can then manually remove them.
