Ours did the same thing for a couple days. Now anytime I walk by they think it's feeing time. Or when I pull my phone out to take pics they swim in front. Should be ok. The first day out black was on the sand not doing anything when I turned the lights on. Few minutes later he was good. Just hadn't had his "coffee" yet.
My pair did that too...they hid at the top by the heater for a few days. Now they are the last to "go to bed" at night and the first one out in the morning. And they always come to the glass when I walk by
__________________ Don't try to fix something that isn't broke. You just might end up breaking it