How is the fish doing today?
Whenever you post an issue, it is always best to also list background on the tank and critter that you are having issues with. Having a picture is absolutely helpful, but now we have to spend sometime finding out the background to help to find what is wrong and the solution. Having this info right up front can help you get answers faster from those who were on the site with you yesterday.
So, things we need to know.
-- Is this a new fish?
-- Your water parameters of ammonia, nitrite, nitrate
-- Did you recently make any changes to the tank - in the last month? Any at all?
-- What other signs/symptoms do you see that is out of the ordinary? Behavior, breathing, appearance, etc.
You mentioned behavior is different. This may not be anything to worry about right away, but something to watch. Is the fish disoriented acting or sluggish?
The picture looks like there is a whitish film towards the back of the fish, but it is hard to see. Is there anything wrong with the tail fin, maybe frayed? Again, hard to tell.
Info above from Glenn with links to Lee are very helpful and I suggest you read over them.
Links off site to read up on:
Brooklynella hostilis
Clownfish are really susceptible to the parasitic protozoa brooklynella hostilis. Slimy appearing aggregate white filmy material on the body is most often Brooklynella, but can be Amyloodinium, Uronema, other protozoans. Read more on Booklynella
here.
Amyloodinium ocellatum
What is looks like is a fine dust appearance, white to tan in color is most often Amyloodinium, though other protozoans can appear this way as well.
here is more on it.
Here is also a very good site on fish diseases.
The more info you can provide to my questions above can help us help you.