Some sponges are not as porous but if it is smooth/slimey, then it is a tunicate. Either way they are harmless filter feeders and their survival/reproduction is an indicator of a healthy tank. There are predatory tunicates but yours look nothing like them and they are typically found in the deep sea where all the other creepy (but very cool) stuff lives 
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Not sure if there are more on the other side of the leather but neither of the ones in your pic appear to be on the coral but rather on the rock. The big black spot in the upper right (on the leather) is a hermit crab right?
.Not sure if there are more on the other side of the leather but neither of the ones in your pic appear to be on the coral but rather on the rock. The big black spot in the upper right (on the leather) is a hermit crab right?
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