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Can't find it on the Metroid Wikia nor wiki.supermetroid.run.
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So apparently some sources say they're just puyos:
https://gamefaqs.gamespot.com/snes/588741-super-metroid/faqs/70645?page=2#Maridia
http://metroid.wikia.com/wiki/Puyo
https://wiki.supermetroid.run/Puyo
Correct. They're Puyos. These grey ones just happen to take very long to start bouncing.