I doubt that this is a crustacean.....

Could you please present your arguments for and against on the other image though please.

It is difficult to say from the pic... But if the picture was taken from a tide pool with saltwater it would be less likely to be an Ephydridae, although they do not mind salinity... The important thing would be to have a lateral view, to see if this has legs or not. Ephydridae larvae often have two long tubes at the end with the posterior spiracles. But I have never seen a larvae with such long tubes...

And I think it is the same species as

I added a zoomed in look at the antennae in both individuals. In the green specimen which is undoubtedly Valvifera we can see a basal segment about half the size of the head, 3 larger segments about the size of the head, while the ultimate half of the antennae is multiarticulate. Compare this to the brown specimen, and although the image is much worse, I can make out the same segmentation with the same proportions. Compare:

Additionally in this image, we can make out a black eye on the side of the head, and though I might be stretching it, I think I can see a faint sign of the max palps as well (Compare to max palps and eye position on the green individual:)

As for the legs, if I zoom in fully and look at the ventral image, I can make out what appear to be seven pairs of hook-like legs folded under the body, one pair per body segment. The legs are transparent, making them rather difficult to see, but compare them to the legs of the green individual, which are also hooklike, transparent, and roughly the same dimensions:

yes this is convincing... being a Dipterist, my first thought is of course a fly ;-) But I was surprised about the length of it... that is too large for Ephydridae....
Thanks for the discussion
Martin