Probably a land flatworm (family Geoplanidae), and probably an alien species.

It is always difficult to provide an identification from photographs, but in this case it could be a species of the geoplanid genus Obama, possibly Obama nungara. 

Please put it in molecular grade 100% ethanol as soon as possible. These animals are fragile.

See Wikipedia:

https://en.wikipedia.org/wiki/Obama_nungara

It looks a bit thick for a marine or freshwater flatworm. The colour pattern looks like Mariofyfea adventor, one of the few exotic landflatworms known from the Netherlands. Ronald Sluys (Naturalis) and Hugh Jones would be highly interested to known and willing to help with ID.

Ook Henk Mienis (Israel) kan helpen, hij schrijft regelmatig over landplatwormen in Nederland en elders http://israel-nature-site.com/wp-content/uploads/2013/12/Natuurhistorische-and-andere-notities-2.pdf

Het zijn iig niet de twee redelijk algemene soorten, Deze site ken je ws wel: https://www.naturetoday.com/intl/nl/nature-reports/message/?msg=23202

groeten, Tello 

Mariofyfea was also suggested by Marta Riutort (Univ. Barcelona) but noted that ID from photo is difficult. The photo at the top on naturetoday.com is in fact Mariofyfea and shows similarity with our specimen. Ronald Sluys is the one that described M. adventor with Hugh Jones) but he hasn't responded yet. Obama nungara also shows similarity with our specimen but I hesitate to believe that, for that species usually shows a median stripe......but you never know. Fortunately the collector (Cor van de Sande) had put the specimen in ethanol :-)

Just tell us the size of the specimen...

Obama nungara: 5 to 7 cm;

Marionfyfea adventor: 8-10 mm.

From the photographs, I guessed it was a big animal.

the collector told me that it was 1-1.5 cm long

1-1.5 cm long... so it's not Obama nungara.

Well, it will definitively need molecular analysis!

Could it be (juvenile) Arhurdendyus triangulates? Not very familiar with them but an important invasive in the UK.