Answer: Based on your description I am not certain what that could be. One possibility however is that a slime mold of some kind has developed, either on the mulch or in some of the organic material in the potting mix they came in or on material used as a soil amendment. You might try gently cleaning it (the spongy material) and the existing mulch away from the area, replace the mulch, and see what happens. Slime molds will actually disappear on their own eventually, but can be rather unsightly. I would also rather see the stem area kept clear for air circulation. I'm sorry I can't be more helpful.
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