My Hammer reached out and killed a Mushroom in my tank also. It was amazing watching it do it. I've been to several aquariums that have saltwater tanks and I can tell you with 100% certainty that hammers, frogspawns, etc. can grow to the size of BASKETBALLS. In constantly checking mine daily to see if I need to harvest it if it starts to take over.
Currently my hammer is surrounded by a Green Spotted Mushroom, Finger Leather, Zoas, & A Bubble Coral all within 6 inches of the edge of it. It dosen't seem to reach out for them. But when I tried to save my Kenya Tree and moved it near it, it started stretching instantly.
What im thinking is that certain corals release a chemical that bothers it and it attacks it. My torch is well above the rest of my tank inhabitants and with water flow blowing it towards the glass.
My Frogspawn is next to some buttons and the red mushrooms that the hammer hated and they seem happy together.