Green encrustation on billon denier tournois

I've recently bought this medieval little coin, as I liked the observe.

Ruler: William I de la Roche (1280 - 1287), or Guy I de La Roche (1287 - 1294)
Denomination: Billon denier tournois, minted: 1280 - 1294
Obs: :G : DVX:ATENES: ( : = double trefoil stop, trefoils resembling Y), cross pattée; rev: reverse :ThEBE:CIVIS: ( : = double trefoil stop, trefoils resembling Y), castle tournois with open corner circles, surmounted by cross, distinctive style
Weight: 0.55g; Ø:1.7
Metcalf Crusades 1030, Malloy Crusaders 85

There is some green encrustation on the coin, both the observe and the reverse. I have two hypotheses:
1) it's the copper in the billion coin that is oxidating
2) it's the result of some PVC holder of the previous owner

How would you commence removing it? Would you? For option one, a low dilution of citric acid may do the trick (risking a pitted surface); for option two, aceton may be best suited.

The next question: this coin is somehow related to the crusades. I lack the knowledge to see why. Who could give some insight?

 

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The de la Roche family ruled a vassal state in Greece after the fall of Constantinople during the 4th Crusade.

 

Okay, so upon further inspection by daylight, I decided to give it a very short citric acid bath (<30 secs at ca. 80 Celcius). Quite happy with the results:

Before:

After:


Thanks @swish513 for the info on the De La Roche familiy.

 

Wow, that's quite the difference! Nice work! Good job erring on the conservative side.

 

Excellent work on a very nice coin.

 

Thanks @seth77. This was one of my earliest coin restoration experiments. The latest was this one:

COUNT OF HOLLAND, Willem V. Denomination: AR Hollandse Groot, minted: Dordrecht, the Netherlands; 1378-1388
Obv: +GVILLELMxDVXxCOMxHOLAxZxZELAND. Helmet in quetrefoil between two Dutch-Bavarian coat of arms, with one ring in each angle
Rev: +BENEDICTxQVIxVENITxIxNOMINExDNI, floriated cross
Weight: 2.02g; Ø:26mm. Catalogue: Grolle 17.9.2A. Provenance: Metal detecting find Oud-Zevenaar, near the assumed location of an 8th century church. ; acq.: 04-2020
Very small flan crack at 6 o clock on the reverse

The coin had several problems:
1) iron oxide depositions, mainly on the obverse, but also on the reverse. These are difficult to remove (I probably need to resort to stronger acids)
2) hornsilver depositions, mainly between the legend

 

Late medieval Netherlands coinage is always interesting.