One of the things that makes this hobby so amazing are all the wondrous and strange creatures that can crop up in their tank and even pests or potential pests can bring a measure of fascination.
One of these enigmatic creatures is Zanclea Margaritae, a hydroid that lives on Acropora corals and is described by scientists as a commensal symbiont.

Acropora colony heavily infested with Zanclea Hydroids

Very little is known about these hydroids and reports of encounters in aquaria are far and few in between. Accounts vary significantly as to the nature of Zanclea hydroids, with some aquarists reporting decline in coral health and loss of the colony while others report them as being harmless and neither being of benefit or detriment to the host coral. Zanclea hydroids have also been reported to wax and wane and ultimately vanish over time in captivity for reasons unknown.

To date I have observed the Zanclea infesteation on several of my Acropora fragments for about a week. As polyp extension and growth seems to be good I have elected to not take any action against the infested colonies for the time being and will continue to observe them in the interest of science.

Further reading: Zanclea hydroid is an Acropora and stony coral parasite you’ve probably never heard of