looks like an sarcophyton latum.
did you move the torch coral on its right (or whatever that LPS is) closer to the sarco in the same time period of these symptoms?
in the fts, the LPS looks farther away from the latum than the closeup of latum retracted with the blotches.
if you did move the LPS closer, try moving that back.
if you didn't, try moving one or the other so that the LPS can't touch the leather. the tentacles/polyps might not be actively touching it but its trailing stingers may be at night or maybe ejected nematocysts(?).
after either change, i would gently scrape the white blotches to see if they come off (clean dowel rod, chopstick, or something similar). i would also make sure the sarco is under some water flow (not strong flow, somewhat moderate and intermittent if possible). hth
