white sponges??

[cathyg_99]

so i have some white sponges growing in my overflow on the pipe and glass, they all are tiny 1mm wide 2mm high and all have holes like in this picture http://www.livingreefs.com/attachmen...e-dsc03256.jpg

the second one is on a rock and is slightly yellow
http://www.chucksaddiction.com/sponge4.jpg

any idea what they are? and if there good or bad??