Is it a sea squirt

This has been in my tank from day one, It's gone a bit bigger and changed colour. The opening at the end does seem to close every now and then. But over the last day or two i have noticed quite a few more of them, they are all transparent. I just hope it's not some sort of parasite :-[

 

the purple one could be but its hard to make out in the pic.

the white thing is a crustation i would say

 

It's the only pic i could get, with my tank being bow front i can only get a clear image through the side. It's only about 3-4mm in lenth. Do flatworms move about in the tank?

 

it's a tunicate/sea squirt with a pod climbing on it

 

Thanks, i was getting worried a bit, i have found maybe 40 more of them. They seem to be near the bottom of the tank in the crevices and hollows. It's an amphipod climbing it, got a few big ones.