Bifferwine
I am a girl
Yep, those are definitely flatworms. They are toxic... So get rid of them now before they have a chance to spread even more. If you wait to get rid of them, they could kill everything else in your tank (when they die, they release their toxins into the water).
In the future, you'll need to be careful about adding new corals. Where did you get those mushrooms from? Since the flatworms seem to be clustered on the shrooms, they likely came with the mushrooms, wherever you got them. If you got those mushrooms from a store, you may want to find a different store to shop at, since flatworms are a very dangerous pest to have in a reef tank... And stores should NOT be selling livestock infested with flatworms. In any case, doing a freshwater or coral dip on new corals will keep the flatworms out of your tank.
So for now, go buy some Flatworm Exit (because they're probably not isolated to just your shrooms any more, unfortunately), treat the whole tank, follow the directions carefully when doing so, and have a lot of water ready to do water changes as soon as you are done with the treatment. Also have a very fine net or powerhead hooked up to a tube with mesh at the end ready to suck up all the dead flatworms as soon as they are released into the water column. The trick is to pick up as many of the dead bodies as you can, so that their bodies don't poison your water.
In the future, you'll need to be careful about adding new corals. Where did you get those mushrooms from? Since the flatworms seem to be clustered on the shrooms, they likely came with the mushrooms, wherever you got them. If you got those mushrooms from a store, you may want to find a different store to shop at, since flatworms are a very dangerous pest to have in a reef tank... And stores should NOT be selling livestock infested with flatworms. In any case, doing a freshwater or coral dip on new corals will keep the flatworms out of your tank.
So for now, go buy some Flatworm Exit (because they're probably not isolated to just your shrooms any more, unfortunately), treat the whole tank, follow the directions carefully when doing so, and have a lot of water ready to do water changes as soon as you are done with the treatment. Also have a very fine net or powerhead hooked up to a tube with mesh at the end ready to suck up all the dead flatworms as soon as they are released into the water column. The trick is to pick up as many of the dead bodies as you can, so that their bodies don't poison your water.