Board Thread:Questions and Answers/@comment-26191333-20150313040509/@comment-4630033-20150313092059

The coin (or in the case of Ethereal Island, shard) sign shows up whenever a monster has earned a certain percentage - I'm not entirely sure whice percentage it is, but this is most likely to say "A monster is almost full on coins/shards, pick them up now or they'll stop earning more for you."

I hope this helps.