Spider-looking creature in my tank?

[FishFinatic ;)]

I have a 20 gallon tank with a 2 month old mandarin goby in there and today I was looking and saw this spider-looking almost translucent creature hanging on the side of my tank almost up against the glass.... does anyone know what this thing is? And is it okay to be in there?

I attached a picture and I know it’s kinda hard to see... it’s the little thing that’s circled. Again sorry for the picture quality....

 

[KJ]

Aiptasia remove them.
http://www.liveaquaria.com/PIC/article.cfm?aid=294

 

[albano]

Looks like aiptasia anemone... get rid of it and any more that you may find.