It sounds like bristleworms as Musho says although thats a blanket description for a variety of the things. The common ones are a useful addition to the tank as they feed on waste material on the sandbed. They often live in the holes in live rocks or bury themselves underneath large shells and the like.
They do pack a punch, so don't try picking them up by hand as you will get stung as a result. They aren't a threat to fish and most will leave them alone. There are some fish that will eat them, although which one's do escapes me at the moment. The worms to watch out for are Fireworms. These
will harm corals and clams. It is most likely that you do not have these, but make sure you get a positive ID on the ones you have and if you spot any that are different, be sure to capture it and ID it.
With regard to ich, there are several options available to you.
#1 is you get yourself some cleaner shrimp or fireshrimp. These will help by picking the parasite off of the fish. Its advisable to introduce a garlic enriched food source to help keep the fishes immune system in check during this period, as the parasite gradually weakens the fish.
#2 is that you quarantine the fish in another tank and treat the water with medication. Do not add copper based medications to you display tank as it will kill invertebrates and corals dead! Raise the the water temperature in the display tank to 82F. This will speed up the life cycle of the parasite. With no fish in the display tank, the parasite will be unable to find a host and will essentially become extinct in the tank after a few weeks.
#3 is the same as the above but instead of medicating the water, you use hyposalinity, whereby the SG is gradually lowered in the QT. This results in the parasite being eradicated as it is unable to survive in the reduced salinity water. This process has to be done gradually and is probably not best attempted by the novice without experienced guidance from a fellow marine keeper.
4# If you live in the US there is a product called "No-Ich" (think that's right). That apparently is safe to use in the display tank and works very well so I have been told. If you live in the UK like me.... you're outta luck on that one lol.
5# You could add a UV sterilizer to your tank if you do not already have one. This also serves to kill the parasite and any other bacteria in the water that passes through the unit. It doesn't guarantee an ich free tank, but it can go a long way to preventing an outbreak.
6# You could invest in an ozoniser. These aren't cheap and much research should be put into the use of O3 before adding a unit to your aquarium. They can potentially kill everything in your tank and be harmful to your own health if over dosed. Ozone is a powerful oxydizer and is used to keep water crystal clear. It can also aid skimmer efficiency too. Because of the O3 in the water any bacteria or parasite it comes into contact with gets nuked. This also means the good nitrifying and denitrifying bacteria too... which is at the hub of most FOWLR and reef tanks eco-systems and is why much care must be taken when using it.
Just out of interest, which fish do you have that are infected with the parasite? At a wild stab I'd guess a tang lol
