plastic thingy with my plants

The plastic bins are for anchoring your plants. I find them helpful because plants tend to float all over the tank otherwise. You're supposed to bury the bin so it won't be seen anyway, although it sometimes manages to poke through. If you think you can anchor the plants without the plastic bin (which is a bit challenging) go ahead and take it off.

Is this like a plastic pot thing with fuzz in it? If it is, don't plant it like that. Remove the pot and take out all the rock wool, then plant it. Otherwise you will put all sorts of stuff you don't want into your tank.

If it's not the plastic pot with rock wool, then please disregard this post

Roan

im gonna go with the first reply and guess it's the anchor- fill that with your sand or gravel.