Hitchhiker ID Needed

[goosey]

Added some live rock rubble to my sump more than one week ago. Found a surprise critter tonight. Whitish/clear with green tinge, soft with bumps on back. Appears to be attached by legs, but I can't easily remove it. Couldn't find its mug shot on the hitchhiker ID thread. Is it good, bad, or indifferent? He's a bit creepy.

 

[bonebrake]

Based off of looks alone it looks like a ball-type of sponge. If you truly see legs then it isn't a sponge.

 

[mie]

It almost resembles a horseshow crab.

 

[groupergenius]

Does it move??

 

[goosey]

It seems to retract a little with pressure. The "legs" seem to be extensions of the main mass as opposed to articulated legs. It has not changed position on the rock.

 

[yosemite sam]

It could be some sort of sponge. Does it have any holes or pores on it's surface?

 

[goosey]

I cannot clearly see pores - it's only 1cm, but it is textured top and bottom (as far under as I can see). The extensions which are holding him to the rock are on the right side of the picture, but on the left side the extensions are finer/filamentous and not attached to rock (almost like antenae). Would a sponge spontaneously grow in minimal light (it's under the cabinet)? Can I assume that because he is not moving that he is unlikely to wreak havoc if I put him in my DT?
It would appear that he has a 3mm sibling developing above him in picture.

 

[bonebrake]

Yes, sponges like that tend to grow in total darkness - under rocks or in overflows where there is no light.

 

[goosey]

Great. I'll leave him in the darkness of the sump. Thanks!