Are sponges good?

[k_jenkinson]

I've been told that i have sponges in my tank but unfortunately I don't know what they actually are or what kind it is. They are lime green and look like it's got a mouth. I just wanted to know if they are good or if it's something i have to worry about. They are multiplying and i haven't done anything different to my tank but they seem to be going out of control