So it looks like I have a ball anemone. I took a picture yesterday and one today after I fed it for the first time. I saw it open yesterday for the first time. Before that it looked like someone chopped a large aiptasia and left it's stump behind.
My guess is that this is a coral of some sort. It looks like a zoanthid. At first I thought it might be an anemone however there's a bud on the stem. This is the first time I've seen it open since I got this particular rock. The tentacles are white currently but hopefully whatever this is can...
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This is the thread where I got my ID. Here are the pictures. Note there's an aiptasia close by but it hasn't grown much since I've had this rock. The rock has been in my system about a month now. The first 2 pictures are from yesterday afternoon. The last 2 are from about 6:30 this evening after I fed it. Tomorrow I'll try to get a picture while it's eating. I wanted to make sure it would be able to hold its food and eat it completely since I fed it slightly too much. I think it's borderline so hopefully it'll digest its food ok. It will be fed less tomorrow.
If this actually turns out to be a genuine giant ball anemone I'll start a new tank for it. If it's a smaller ball anemone it should be ok according to my research thus far. Also people report that their smaller ball anemones hand out in caves and overhangs. Mine is about as much in the light as it can get. I read of a diver who always saw the giant ball anemones basking in the light. It will be interesting to see what it ends up being.