Morning Glory - Heavenly Blue

Planted these seeds in the GH. Came up rapidly and once roots showed at
bottom of seed tray, transplanted them into individual pots about a month
ago. All has been fine, rootball beginning to show at edges of pot, top
growth starting. But over the past week, as the top growth has opened into
leaves, the leaves are pale and yellow. Is this chlorosis? They are still
in the GH and have some shading.

Any thoughts as to the cause? And how to remedy it?

TIA
--A