The greyingout of certain applications after moving from or to a Sd card can happen because of multiple reasons.
One, the card is or getting corrupt ( shutdown phone, remove card, stick it in to a pc/laptop, go to properties and do a check ).
Second, not al apps are written ( or allowed ) to work from a sd card.
Third, there are apps who uses or stores some extra data in the secured partition, whenever those apps are moved the link to the data gets or can be corrupted......so the apps crashes or gets grayed out ( these apps are mostly the apps who are quite difficul to delete after a recovery, update or reset.).
If you focus on the third...............it come to notice that a lot of those apps who reacts like that are build for older O-systems like wp 7x or 8.
I say a lot, not all...( just to prevent a discussion )......