I started to panic when I saw mine do it for the first time too.
When I first brought him home my Clarke clown pounced on him like a dog with his favorite toy.
It fed him, cleaned around him and everything looked great.
Then about a week later it started.
At least once every day or two it would shrivel up, it's mouth would gape open sickeningly and long strands of dark slime would come oozing out.
I could've sworn it had gotten deathly ill and was on it's last leg.
I read everything I could find on anemones expelling slime and frankly, most of what I read was alarming and scary... and turned out to be completely false.
After going thru multiple water changes, adding all kinds of suppliments, changing the lighting from 4wpg to 10wpg, changing the food, changing filter type, adding an intank fuge, changing substrate, changing water flow, isolating him from the clown, etc, etc, I finally found out that he wasn't dying and he wasn't losing his zooxanthellae, he was just pooping.
Sometimes I see him poop everyday or so... sometimes I don't see him poop for a week or more... but one thing's for sure, he's done nothing but grow bigger and get darker coloring since the day I got him.
So long as your little guy doesn't start getting bleached out and always reinflates to a healthy looking shape after deflating for a while then I think it's safe to say he's just doing what comes natural.
I couldn't remember the link to her page. (bookmarked now)
Her's is one of the better sites with a lot of good info on anemones.