Disappearing spots on firefish

[Mebbid]

In my 20g saltwater tank I have a Firefish and Scooter blenny along with a few peppermint shrimp, turbo snails, and hermit crabs. My firefish randomly gets small white spots on him which disappear in around 12 ish hours. For the most part the spots show up at night and are gone by the morning. So far this is the second time that the spots have shown up on the firefish with the second time being much worse. I would attach a picture but the spots are gone now. I've had both of the fish for about a week now. Along with the fish I have both live sand and live rock. Both of the fish were drip acclimated over a few hours because of a difference in salinity.

Both of the fish ravenously eat frozen Mysis shrimp and are very active in the tank. The firefish doesn't seem to act any differently whether he has the spots on him or not. The scooter blenny doesn't seem to have any of the spots show up on him, although he naturally has quite a few similar sized spots on him so I might not be noticing them. Stress coat is the only additive I have used in the water.

Temperature - 78 degrees
SG - 1.024
Ammonia - 0
Nitrite - 0
Nitrate - 5

In my tank I have:
Live Rock - 18lbs
Live aragonite Sand - 25lbs
90g Sponge filter
175gph powerhead
Protein Skimmer

It looks a lot like Ich to me but it's confusing because the time frame for the spots showing up and disappearing makes absolutely no sense to me.

[Beengirl]

I don't see a pic, but are you certain it's not just sand? Firefish tend to burrow in the sand and sand could get stuck to them for a little while and then fall off later.

[okapizebra]

I've experienced something similar when I first got my firefish. At first I thought it was ich, but it went away just like yours and didn't seem to be bothering the fish at all. You said you've only had him a week? I'd just give him some time to adjust to your tank. As long as he appears healthy and happy, I wouldn't be concerned.