Those look like brown flatworms. They are considered pests. The main problem is that they are blocking light on the coral and some believe they are feeding on the zooxanthellae from the coral tissue. Different types of flat worms are very common. Some bad. Some not. I'd say all of them on your mushroom like that is not good. You have several options here. Natural predators for these are six line wrasse's, dragonettes etc.. in addition to good water quality. They will thrive in high nutrient waters.
They are sensitive to salinity changes.. more so than your coral. So for the immediate I'd say to pull that mushroom and give it a fresh water dip in RO water for 10 seconds and shake the mushroom around. They should just fall right off. Just make sure that your Ph and temp is as close to the tanks as you can get it.
You want to try and get all of them off if possible. Flatworms are toxic when they die so if you are using activated carbon in your system then even better.
No problem. If you see this many on your mushroom then there are certainly more in the tank. So this will help for now but you're going to want an ongoing solution to the problem. Like clean water and a natural predator. More will likely show up on the mushroom again.