Unless your temps are very high, a 53% RH is fine .Hell in winter I'm lucky to get up to 30% RH and my plants don't have issues because I run a lower temp, 68-701F. Might have slightly less growth but not enough to really notice. At RH 53% your withing a normal range if your temps are acceptable. That is Mg deficit on his leaves. It may be caused by over feeding from the color of his leaves and early signs of clawing, the Mg def could be a side effect of over feeding
Was 53 and when my little humidifier runs out(16oz) goes down to 56-57, but 74% is the high end. Need to fill a few time a day. So much higher now. May still be a deficiency not 100% sure. Sure looks like calmag deficiency in pics cant find toxicity pics, although some say deficiency and excess calmag so may look the same. Still learning everyday hope that's it easiest fix.