Orange seedlings: Weird leaf discoloration

[flyingfishfinger]

Hi,
Of my 4 seedlings, 2 had some scale-like issues as noted in the post currently below this one. The third one has a different issue, as attached. I'm new to citrus trees, so I have no idea what this might be about... any thoughts, please? All 4 are next to each other, indoors, at a sunny but west-facing window. Is starting to remind be a bit of HLB, could that be it?

Cheers,

R