Hydroids are tiny jellyfish-like creatures. These in particular are in their Medusa stage. Typically they're seen on relatively new tanks, or perhaps they hitchhiked in from something or the other (live rock, frags, etc).
These are typically not of anything to have an emergency situation with, but they do need to be monitored closely such that they do sting surrounding inhabitants.
Hydroids feed off of particulate matter and nutrients so ensuring you have good husbandry and aren't an over feeder, I would recommend keeping up those good habits and keeping an eye on the hydroids.
If you were interested in removing them, then Fenbendazole is a known medication that would assist in that - but I would urge you to consider other approaches before reaching for the pill!
Best of luck to you!
