Is this the centre of the stem of a mushroom or different kind of mushroom?
This does look like the centre of the stem. Often when I separate the stems from the heads, the centre of the stem would stay attached to the head, It looks just like this.

In any case, if you're buying your mushrooms from a reputable source, you can be sure there are no poisonous mushrooms accidentally mixed in. Cases of mushroom poisoning occur mostly when people gather their own mushrooms and mis-identify them, (or buy from someone who gathers wild mushrooms and has erred with identification,) or, more rarely - when one allows mushrooms to go bad.