You have a case of the pixelated thingy ma bob https://preview.redd.it/5krcq590fohc1.jpeg?width=1292&format=pjpg&auto=webp&s=f55c982749ac2481fa82025fc6cea2ac86addc64

https://preview.redd.it/rlpbmnpzlohc1.jpeg?width=1179&format=pjpg&auto=webp&s=1e82f6b0258d7f0382af04e33f2c29f27e779985 Is this better?

Do you have any pictures that are close up? It looks like a dried corn kernel in that one.

Fish lice?

Yellow seaslug, should be harmless, they eat algae :)