Looks to me like a Heteractis Malu. If that's the case they tend to prefer sandy substrate, so try putting in in the sand.
Details Tentacles arise from brown or purplish (rarely bright green) oral disc as much as 200 mm in diameter that may have white radial markings; evenly tapered to point or slightly inflated in middle; lower part same colour as oral disc, but upper portion may have several white rings or green end. Column very thin in expansion; upper part violet-brown (due to zooxanthellae) with longitudinal rows of adhesive verrucae. Anemones can retract completely into sediment; most common in shallow, quiet waters.
Another condy vote.
Definitely not a malu.
It doesn't fit that description. The tentacles are too thick to begin with and then evenly thick and too long. A malu of that size would have proportionally shorter tentacles.My guess of Malu is based on this description:
"Details Tentacles arise from brown or purplish (rarely bright green) oral disc as much as 200 mm in diameter that may have white radial markings; evenly tapered to point or slightly inflated in middle; lower part same colour as oral disc, but upper portion may have several white rings or green end. Column very thin in expansion; upper part violet-brown (due to zooxanthellae) with longitudinal rows of adhesive verrucae. Anemones can retract completely into sediment; most common in shallow, quiet waters."
Source: http://jjgeisler.com/reeftank/anemones/ch1.htm