Are you limited to a phone? That would be very tedious with any instrument and probably impossible on a phone. It is not the simple removing the square shadow but bring (clone) one of the other trees over to blend with the existing fronds protruding from the square shadow AND provide the detail where the shadow was removed. Actually, I would mix from both trees so doesn't look like have two identical trees.
If you don't have a photo program on your computer, consider Gimp which is free. The Mac version hasn't been updated yet, but this is so basic of advanced procedures, probably in the earlier version. While I use Photoshop, as far as other programs, other than free trial, probably the least expensive is Affinity Photo. With it and its competing products, they will have 'how to' videos on a learning/education page of their website. Look for a video on cloning. If they don't have one...it probably can't do it.