Strange Growths

Just recently in my 55gal tank some strange tubular growths have occured. They start green at few millimetres then grow. At about 10 mm the top becomes bright luminescent green. At this stage you can see the top has small tubes/filters. Eventually they start to calcify (I think) from the base upwards. There is a picture of full grown one in the gallery under "quantumfoam" album. My guess is some type of sponge (syconoid or asconoid) or a Cnidarian of some sort. Anybody know what they are??

 

Neomeris annulata

Usually called caterpillar weed. It's a calcareous algae just like Halimeda. In other words, if you have too much of it, your Calcium levels will drop.

 

Many thanks. Will keep them at minimal level.