There are lots of reason why SPS would lose their flesh. If you have a large colony with dense branches, light and flow will be hard to reach inside the coral which could cause tissue lost. This is very common even for an otherwise healthy acro. If you have SPS predator (such as AEFW), it's possible the flesh is being consumed. If you have a alk swing or water quality issue, it would cause RTN either from the tips, bottom up or anywhere else. Obviously, if you have a pest anemone physically in contact with the coral, it will most certainly cause tissue lost as well around the contact point. The possibility is endless and lots of times we simply don't know why which force us to physically remove the affected area hoping to save the rest.
Depends on the actual cause, the tissue lost might or might travel up the body. For example, if your coral is healthy and growing (like you mentioned) and the tissue lost is really caused by the anemone stinging it, the coral will recover once you remove the pest. If it's caused by other reasons such as light, flow or water quality issue, it's very likely the condition will continue to worse until you fix whatever problem it's there. If the anemone is your first suspicious, I would just remove it (you will have to before it spread anyway) and see if the coral recover. Is this the only SPS you have? Have other SPS show signs of tissue lost? Have you check the usual alk / cal / mag just to make sure they are stable? Have you dip the coral or inspect closely for pest?
