|Yes, this expansion is normal, healthy behavior for corals in the genus Trachyphyllia. In nature, these corals live unattached on the sandy substrates near reefs. In order to change location with the current and/or eliminate sand and debris that has settled on the fleshy part of the coral, they often take in large amounts of water to increase their size and buoyancy.
To keep your Trachyphyllia healthy, allow it ample room to expand in your aquarium. Place it on the substrate away from other corals or invertebrates that may sting or irritate it. If the Trachyphyllia expands into other corals or inverts, relocate it to an area where it has sufficient room to expand. If your aquarium has low lighting, your Trachyphyllia will thrive, but will also expand to maximize its exposure to light. Beginning and advanced reef aquarists alike enjoy Trachyphyllia for its intrigue, ease of care, and ability to fluoresce beautifully under actinic lights.