First off, any unexpected tank inhabitant never really tends to be a good thing, especially when you don't know what they are and how much potential damage they could cause to your aquarium and the community living within it.
For the Hydroid Jellyfish, it is not something that you particularly want to be residing in your tank. However, it is certainly not the worst critter to find in there. They are not freshwater jellyfish, so the chances of finding them in a freshwater tank are incredibly unlikely.
How To Identify Them
If you do discover these tiny organisms that somewhat resemble small stars due to their odd tentacle and body shape sticking to the glass of your tank, that would be the Hydroid Jellyfish. They go by other names too, Cladonema, Hydromedusae & fixed Jellyfish. They kind of look like a starfish crossed with a snowflake, but are not that big at all, with the maximum size being around 1 to 2 mm.
These small Jellyfish tend to be discovered in new and less established tanks, usually by beginner hobbyists who may not have a clue what they are and what they are doing there, which is totally understandable! They will typically be found in a group of maybe 4 to 6 and then, as time moves on and your tank ages, you will likely see them disappear.
Do They Sting?
Most of the time these Jellies are relatively harmless and won't cause much of an issue for the rest of the community. However, like most other jellyfish, they sting, and will likely sting if contact is made. Fortunately for us hobbyists, the jellyfish sting won't hurt us, and we will likely feel like nothing (unlike a Box jellyfish). Your fish, however, certainly will feel it, especially if it is a small fish. Now, for the most part, they will not get hurt or injured, but it will be uncomfortable for them and can lead to distress.
Our advice, and also other people's experience with these Jellyfish is that if they are in small groups, they are generally fine and will not cause much of a disturbance to your community and aquarium. Although, if the numbers rise then they can become somewhat of a nuisance and will need to be cleared out.
An old wives’ tale is that certain species of Wrasse eat the Hydroid. However, there is no evidence of this, and you certainly shouldn't bank on it! To be honest, there is no real method in tackling a large scale of Hydroid Jellies, the best advice would be to build your tank to a more established level and by then most of them will have vanished.
On the topic of the Wrasse eating them, it leads to a common question when discussing the Hydroids, do they have any predators? Unfortunately, or fortunately - whichever way you want to look at it, they do not have any known predators, however, some people have found the Six Line Wrasse as an effective predator for them.
The issue with introducing a potential predator like the Six Line Wrasse just to take care of your Jellyfish problem is that you then have to make sure that your tank community will live in harmony with that specific fish, and also whether or not your aquarium can consider the Wrasse’s requirements, i.e., tank size, specific food and feeding, temperature, water flow, and more!
The problem with the Six Line Wrasse is, despite being a beautiful fish sporting awesome colors and patterns, they have a pretty bad reputation for not playing fair with their tank mates, mainly due to their aggressive nature - they are notorious tank bullies. Not only this, but you might find out that your 6 Line Wrasse isn't interested in eating the Hydroid Jellyfish either.
In conclusion, these Hydroid Jellyfish are somewhat of an inevitability when you have a less established tank and are just starting off on your saltwater aquarium ownership journey. The best thing to do is to keep an eye on them and make sure that they don't overrun the place, in most circumstances they won't, but sometimes they do and will cause problems for your community.
If this is the case then it is recommended to just manually remove them, but if you did want to introduce a predator to make light work of them, the Six Line Wrasse is the only fish to really have a reputation for it.
The Six Line comes with its very own set of problems, so it really should only be introduced if you have the right tank and community for it, the last thing you want is a tank with a bunch of Hydroid Jellies that aren't being eaten and a Six Line Wrasse bullying the rest of your fish! It will ultimately be more trouble than it is worth!