Answer: It isn't necessary to remove the rocks; you can just place new mulch over the top. But eventually the mulch will decompose and either raked up or covered over again with fresh mulch. If this isn't a concern for you, go ahead and place the mulch over the lava rocks. Try to avoid piling the mulch up against the main stems of the plants. If the mulch touches the woody stems of your plants it can hold too much moisture against them and cause the bark to rot.
Good luck with your garden!
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