What are the characters in this seal? (Characters identified: 白云長滿目)
The characters are 白云長滿目, which roughly translates to

The vast expanse of white clouds, filling one's sight,

coming from the poem 《送廬山人歸林慮山》 by the Táng Dynasty poet Huángfǔ Rǎn.
「云」 is particularly difficult to identify, and is worthy of some additional explanation.

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「云」 (Baxter-Sagart OC: /*[ɢ]ʷə[r]/, clouds) is comprised of semantic 「」 (up/above, now written 「上」) and phonetic
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(/*[ɢ](r)ə[r]ʔ/). This character is 「旬」 before the addition of a 「日」 component.
Occasionally, the 「」 part of 「云」 may be partially or fully omitted, and the remainder may be rotated or distorted in some way. This forms the inspiration behind
說文古文雲部說文解字
which is the shape in the seal.

References:

季旭昇《說文新證》
小學堂字形演變
國學大師

郭沫若《甲骨文合集》
中國社會科學院考古研究所《殷周金文集成》


宋・徐鉉等奉敕校定　漢・許慎《說文解字》