BTA retreating into it foot?

[Palmer]

Hi all,

Fairly new to anemone's. I have two GBTA's and when I looked in my tank today one of them looked like it was being swallowed by its own foot. About 30 minutes later I looked again and it was normal status quo. For a moment there it was pretty dramatic looking. I wasn't sure if it was stressed by something or what.

Any reason they do this??