Hmm... I can think of a couple of different interpretations of RAW, not sure which is RAI.
You could argue that the contest to resist grappling is a check for initiating grapple and that a check to maintain a grapple is a check for escaping (the rules certainly don't call the individual rolls in the contests by different names).
As DxJxC said, the text of the ability doesn't actually state the role the character has in the grapple situation.
"You have advantage on ability checks you make to initiate or escape a grapple."
It could be read as only rolls when YOU initiate or escape a grapple.
However, when someone tries to grapple you, you are making a roll to initiate a grapple - it is just the opponent that is initiating it.
The sentence could be parsed as:
"(ability checks you make) (to initiate or escape a grapple)"
or
"(ability checks) (you make to initiate or escape a grapple) -> (ability checks) (you make to initiate) or (you make to escape) a grapple
Does the "you" apply to ability checks you make - or does the "you" apply to the latter part of the sentence ... "you make to initiate or you make to escape"? Potentially ambiguous like a lot of 5e rules.
Personally, I'd apply the plasmoid ability to any grapple check since their structure doesn't change so if they have advantage to get away, I don't see why they wouldn't have advantage to avoid being grappled in the first place.
The Spelljammer Plasmoid race has the Amorphous trait:
This could/would imply:
Is RAW the way it was intended? Could it be re-written as follows?
Hmm... I can think of a couple of different interpretations of RAW, not sure which is RAI.
You could argue that the contest to resist grappling is a check for initiating grapple and that a check to maintain a grapple is a check for escaping (the rules certainly don't call the individual rolls in the contests by different names).
As DxJxC said, the text of the ability doesn't actually state the role the character has in the grapple situation.
"You have advantage on ability checks you make to initiate or escape a grapple."
It could be read as only rolls when YOU initiate or escape a grapple.
However, when someone tries to grapple you, you are making a roll to initiate a grapple - it is just the opponent that is initiating it.
The sentence could be parsed as:
"(ability checks you make) (to initiate or escape a grapple)"
or
"(ability checks) (you make to initiate or escape a grapple) -> (ability checks) (you make to initiate) or (you make to escape) a grapple
Does the "you" apply to ability checks you make - or does the "you" apply to the latter part of the sentence ... "you make to initiate or you make to escape"? Potentially ambiguous like a lot of 5e rules.
Personally, I'd apply the plasmoid ability to any grapple check since their structure doesn't change so if they have advantage to get away, I don't see why they wouldn't have advantage to avoid being grappled in the first place.