I would say it's a sponge. The tube with opening is called an operculum. Some of the sponges in my tank are really growing. If they get in the way, I just pull them off. But I find they are no match for any corals - they just stay in the shady spots or under the coral.
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![](https://live.staticflickr.com/65535/48920649641_e570664267_o.gif) Maggie
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