teeny tiny mysid shrimp (maybe)

[Delphinus]

They may indeed be actual Mysis sp. - ultimately just part of your 'pods population - and yes a good thing - and they'll come in on anything - coral rubble live rock etc.

You are seeing them there because it's harder for fish to eat them all if they can't get at them. Kind of shows how things like pod piles can work well though (small piles of live rock rubble with lots of gaps for the pods to establish "safe havens").