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In the white tips you are seeing the coral depositing CaCO3 (calcification) as the skeleton grows at these points. The colour occurs at the same pace as zooaxanthellae grows to cover the new skeleton but it always lags behind. When the coral is growing very fast the white tips will be longer because rate of calcification has increased and zooaxanthellae growth will try to catch up. So it's a continuous process. The shorter/less visible the white tips the slower your acro is growing (No white tips at all in acropora means no growth). A longer white tip is therefore a sign that your acro has adjusted to your tank conditions and is growing fast.
Hope this helps.
Also, the acro stems will be getting gradually thicker to support the increasing weight of the acro as it grows but normally this is harder to notice.
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