By tentacles, do you mean polyps? I can't tell from the photo, but it kind of looks like it's a toadstool.

Both my devil's hand and green polyp toadstool likes a moderate amount of flow over it to wash off the jell that forms over its skin. The white spots... I get that on mine if I touch it or sand accumulates on a portion for a couple of days or so. When I baste it off what's left is a light mark.